Hi luke.smith:
I'm not sure of a way to obtain releases prior to the archived list (which goes back a of couple years). If you have a unique issue requiring a specific release, I recommend contacting ADTRAN Support directly. Although they're not obligated to continue support for very old product and software releases, I always find that the support engineers are eager to help. If you cannot obtain the firmware image through ADTRAN Support, perhaps a work-around or alternative approach to your project/case can be suggested.
888-4-ADTRAN
Best,
Chris