<PAGE>   1
                                                                    EXHIBIT 4.3

                                                                   ATTACHMENT B



                                   STDF CORP.

                      10.25% SUBORDINATED PROMISSORY NOTE

                                  NO. _______

$XXX,XXX                         HOUSTON, TEXAS                   JUNE _, 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 June _, 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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