I think I figured it out?
I think the 77A uses a frame nearly identical to the K8, which uses the same frame as the K7 (mostly). Apparently there's some contention between what luggage racks fit what models, but the safest bet seems to be to assume that if it fits a K7 it fits a K8. Since the...
I'm not sure which frame my bike matches and no one is labeling CB750 luggage racks as "CB750A racks".
I think the F models have a different frame than the K models, but do the K models have a different frame than the A models? Not to mention the differences between the year the bike was made...