You got me thinking a bit more too so I did more investigating and.....I think it might be more of a year thing. I have a '66 A-body w/904 and a '66 B-body w/727 and they have the same "A" bracket but the '72 B-body w/727 has the "B" bracket. The only real difference it the "B" bracket swings down a bit lower than the "A" bracket. I'm not sure when they changed over but there are two versions of the "A" bracket. The early one has a rubber grommet with a metal clip and the later has only a plastic bushing. I'll check a couple more sources and add more info as I get it.
