<PAGE>   1
                                                                    EXHIBIT 4.2


                                                                   ATTACHMENT B


                                   STDF CORP.

                      10.25% SUBORDINATED PROMISSORY NOTE

                                  NO. _______

$XXX,XXX                         HOUSTON, TEXAS                    APRIL _, 1997

FOR VALUE RECEIVED, the undersigned, STDF Corp., a Texas corporation ("Maker"),
promises to pay to the order of ______________________ ("Holder"), the
principal amount of  $XXX,XXX, together with interest on the principal balance
from time to time remaining unpaid at the rate and upon the terms provided in
this Note. Each payment of or pre-payment under this Note shall be made in
lawful money of the United States of America at the mailing address of the
Holder set forth on the signature page to the Subscription Agreement between
Maker and Holder of even date herewith, or as subsequently provided in writing
by Holder to Maker.

1.     PAYMENT TERMS. The entire unpaid principal balance of this Note , plus
       all accrued and unpaid interest on this Note shall be due and payable on
       December 31, 1998 (the "Maturity Date"). Interest on the outstanding
       principal balance of this Note shall be due and payable on the first day
       of each July, October, January and April as it accrues, and on the
       Maturity Date

2.     INTEREST RATE. The unpaid principal balance of this Note from time to
       time outstanding will accrue interest from the date of this Note until
       the Maturity Date (and thereafter until paid) at a fixed rate which
       shall be equal to ten and one quarter percent (10.25%) per annum and
       shall be calculated on a 365 day basis.

3.     PREPAYMENT. Maker may prepay all or any portion of this Note at any
       time, without premium or penalty.

4.     SUBORDINATION. All principal and interest obligations of Maker to Holder
       under this Note shall be subordinate to all other Obligations in right
       of payment. Upon occurrence of a default under an Obligation, Maker may
       not pay the amounts due under this Note prior to payment in full (or
       cancellation as the case may be) of all Obligations. Notwithstanding the
       foregoing, until a default occurs under an Obligation, Maker may pay the
       amounts due under this Note. If Holder receives any prepayment from
       Maker in violation of this Section, Holder shall hold any such payment
       in trust for Maker and shall promptly turn such payment over to Maker in
       the form received (with any necessary endorsements), to be applied to
       the Obligations. For purposes of this Section, the term "Obligations"
       means all obligations of the Maker to all parties other than the Holder
       of this Note and the other 10.25% Subordinated Promissory Notes of even
       date herewith due December 31, 1998 issued by Maker.




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                                                                    ATTACHMENT B



5.     EVENTS OF DEFAULT: The term "Default", as used in this Note, shall mean
       Maker fails to pay all or any portion of the amounts due under this Note
       when due and such failure continues for 30 days after Maker receives
       written notice of Default from Holder.

6.     REMEDIES OF HOLDER. Upon the occurrence of a Default under the terms of
       this Note, Holder  shall have the following rights:

       a.     ACCELERATION: Holder may, at his option, and upon giving notices
              required by applicable law, declare the entire principal balance
              of this Note and the accrued but unpaid interest thereon,
              immediately due and payable.

       b.     OTHER RIGHTS: Holder shall have all the other rights and remedies
              available at law or in equity.

7.     WAIVER OF JURY TRIAL. MAKER AND HOLDER HEREBY WAIVE TO THE FULLEST
       EXTENT PERMITTED BY LAW, THEIR RIGHT TO A JURY TRIAL OF ANY CLAIM OR
       CAUSE OF ACTION BASED UPON OR ARISING OUT OF THIS NOTE OR THE
       TRANSACTIONS COMTEMPLATED HEREBY. The scope of this waiver is intended
       to be all-encompassing of any and all disputes that may be filed in any
       court and that relate to the subject matter of this transaction,
       including, without limitation, contract claims, tort claims, breach of
       duty claims, and all other common law and statutory claims. Each party
       further warrants and represents that it has reviewed this waiver with
       legal counsel, and that it knowingly and voluntarily waives jury trial
       rights following consultation with legal counsel. THIS WAIVER IS
       IRREVOCABLE, MEANING THAT IT MAY NOT BE MODIFIED EITHER ORALLY OR IN
       WRITING, AND THIS WAIVER SHALL APPLY TO ANY SUBSEQUENT AMENDMENTS,
       RENEWALS, SUPPLEMENTS, OR MODIFICATIONS TO THIS NOTE. IN THE EVENT OF
       LITIGATION, THIS NOTE MAY BE FILED AS A WRITTEN CONSENT TO A TRIAL BY
       THE COURT.

8.     REQUIREMENTS FOR TRANSFER.

       a.     The initial Holder hereby certifies, warrants and covenants to
              Maker, under penalties of perjury, that

              (i)    Holder (and any person on whose behalf Holder is acting)
                     is not a United States person as that term is defined in
                     the income tax laws of the United States,

              (ii)   Holder is the beneficial owner of this Note,

              (iii)  Holder is not a ten percent (10%) shareholder of Maker as
                     defined by the Internal Revenue code of 1986, as amended,
                     Section 871(h)(3),

              (iv)   a United States Internal Revenue Service Form W-8 shall be
                     executed by or on behalf of Holder, and





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                                                                    ATTACHMENT B


              (v)    Holder promises to notify Maker within 30 days of
                     occurrence if its country of citizenship, residence or
                     organization is found to be, or changes to, the United
                     States.

       b.     Each subsequent Holder who is not a United States person will be
              required to certify to Maker the information set out in
              subsection 8.a. above by execution of a Transfer Statement in the
              form attached to this Note as Exhibit A in order to effect the
              transfer of record title to this Note to a transferee on the
              books of the Maker. A Transfer Statement submitted to the Maker
              by any transferee who is a United States person shall have
              paragraph B thereof deleted and a United States Internal Revenue
              Service Form W-9 attached.

       c.     Any subsequent Holder who is or becomes a United States person
              shall promptly provide to Maker a United States Internal Revenue
              Service Form  W-9.

       d.     Each Holder shall re-certify the information set forth in
              subsection 8.a. above to Maker annually or as more frequently
              requested by Maker.

       e.     Each subsequent Holder / transferee shall complete, execute and
              deliver to Maker, a Transfer Statement, in the form attached to
              this Note as Exhibit A, without change or alteration, except as
              provided or permitted in Section 8.b. of this Note.

9.     CAPTIONS AND CERTAIN DEFINITIONS. The captions, headings, and
       arrangements used in this Note are for convenience only and do not
       affect, limit, amplify, or modify the terms of this Note. As used in
       this Note, the term (a) "Holder" means "Holder" as defined in the
       preamble to this Note and all subsequent holders or transferees of this
       Note, and (b) "Maker" means "Maker" as defined in the preamble to this
       Note and all subsequent holders and transferees of this Note, and (b)
       "Maker" means "Maker" as defined in the preamble to this Note and its
       successors and assigns.

10.    APPLICABLE LAW. This Note shall be governed by and construed in
       accordance with the laws of the State of Texas and the laws of the
       United States applicable to transactions within such state.





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                                                                    ATTACHMENT B


THE SECURITY REPRESENTED BY THIS NOTE HAS NOT BEEN REGISTERED UNDER THE
SECURITIES ACT OF 1933, AS AMENDED, (THE "ACT"), OR ANY OTHER APPLICABLE
SECURITIES LAW. THE SECURITIES REPRESENTED BY THIS NOTE MAY BE TRANSFERRED
PURSUANT TO AN EFFECTIVE REGISTRATION STATEMENT UNDER, OR IN A TRANSACTION
EXEMPT FROM REGISTRATION UNDER, THE ACT AND IN ACCORDANCE WITH ANY OTHER
APPLICABLE SECURITIES LAWS.


                                        STDF CORP.  
                                        A Texas Corporation

                                        By:
                                           ------------------------------------
                                           Name:  Matthew D. Pond
                                           Title: Chief Financial Officer





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                                                                    ATTACHMENT B


                                   EXHIBIT A
                               TRANSFER STATEMENT


A.     The undersigned Holder ("Holder") hereby transfers and assigns to
       _______________ ("Transferee") all of Holder's right, title and interest
       in and to that one certain 10.25% Subordinated Promissory Note No._____
       dated April __, 1997 (the "Note"), executed by STDF Corp. ("Maker") and
       payable to the order of Holder in the original principal amount of
       $_________.

B.     Transferee hereby certifies, warrants and covenants to and with Holder
       and Maker, under penalties of perjury, that

              (i)    the Transferee (and any person on whose behalf Transferee
                     is acting) is not a United States person as that term is
                     defined in the income tax laws of the United States;

              (ii)   the Transferee is the beneficial owner of the Note;

              (iii)  the Transferee is not a ten percent (10%) shareholder of
                     the Maker as defined by the Internal Revenue code of 1986
                     (as amended, the "Code") Section 871(h)(3);

              (iv)   a United States Internal Revenue Service Form W-8 executed
                     by or on behalf of the Transferee is attached hereto; and

              (v)    the Transferee promises to notify Maker within 30 days of
                     occurrence if its country of citizenship, residence, or
                     organization is found to be, or changes to, the United
                     States.

C.     Transferee hereby further certifies, warrants and covenants to and with
       Holder and Maker that:

              (i)    the Transferee has been furnished with and read carefully
                     the Confidential Private Placement Memorandum dated April
                     1, 1997 (the "Memorandum"), including the Attachments
                     thereto, pursuant to which this Note was acquired by the
                     initial Holder hereof and understands the terms of  the
                     offering of the Notes;





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                                                                    ATTACHMENT B



              (ii)   the Transferee affirms hereby directly to Maker as its,
                     his or her own representations and warranties each of the
                     representations and warranties of a subscriber to a Note
                     contained in Section 3. of the Subscription Agreement
                     included as an Attachment to the Memorandum; and

              (iii)  the Transferee is an "accredited investor" as that term is
                     defined in Rule 501 (a) of Regulation D promulgated under
                     the Act.





TRANSFEREE:                                 HOLDER / TRANSFEROR:

By:                                         By:                         
   -----------------------------               --------------------------------
Name:                                       Name:                       
     ---------------------------                 ------------------------------
Title:                                      Title:                      
      --------------------------                  -----------------------------
Date:                                       Date:                       
     ---------------------------                  -----------------------------


Address of Transferee:
                                
--------------------------------
                                
--------------------------------
                                
--------------------------------
                                
--------------------------------


STDF CORP.:

By:                             
   -----------------------------
Name:                           
     ---------------------------
Title:                          
      --------------------------
Date:                           
     ---------------------------





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