I have put a 5 speed in my GS and decided to see if the owner of the Buick GS that I got the 4 speed from is on the forum. The trans came from Upper pennisula of Michigan around the Travers City area, and was out of a 1971 Stage 1 with this number stamped on the side 41H12796 date 1M04B (#'s look like a 1st grader stamped them) not even, run together, spaced apart. I think a man named Larry, who had a bunch of Buick street rods and GS's is who I traded my auto pedals and console, shifter, and linkage for his four speed setup back in 96, 97 or 98. The trans will only be sold back to the owner of this car if they can send me a photo and proof of ownership, so if you have a 71 Stage form Upper Michigan and want the orginal trans back, contact me. 432 634 5980 or PM me here.
Have you tried Traverse City Craigslist? Traverse City is in the northwest area of the lower peninsula. Also, if the guy is an active hotrodder, check their local car clubs, he may be a member.
May try the Flint area craigslist also because a lot of these cars may end up in the Sloan museum bank of info. They have a lot more records on the 70's Buicks than the previous years.