Okay. So here’s the deal. On page 30 of the 1975 parts section I found the broken part. It’s one of two “ears” that are on the opposite side of the chain adjustment rod. There’s a rod that goes through the two ears. The outer one’s ear broke off down near the hole the rod runs through. It looks like the two ears and shaft locate the sprocket and maybe when you tighten/loosen the adj shaft the whole housing pivots to make that happen. There’s no number or name for the part, although it looks pretty important. It does seem to be aluminum. Not sure what I did to break it, though.