Installer 3.1 had minimal testing with System 7.0. If you are expecting to install software onto machines running System 7.0, you should consider upgrading. Script changes should be minimal. A Use the checkFileVersion clause as part of the 'inrl' Rules Framework resource. The most common difficulties are in remembering that BCD values are required and in dealing with two-digit version numbers.
Some samples follow.
Under System 6, each time I run the script, a new copy of the DA appears as a DRVR resource in the System file. Unfortunately, this is a symptom when the 'deleteWhenInstalling' flag is used in conjunction with the 'updateExisting' flag. The guide also indicates that the Installer will overwrite a preexisting resource in the target file if the 'updateExisting' flag is set. The correct Installer action is accomplished by setting the 'deleteWhenInstalling' flag in conjunction with the 'updateExisting' flag. Alternatively, use the 'dontDeleteWhenInstalling' flag with the 'keepExisting' flag.