Identifying 73 and up disc brakes

Those are 73+ discs. 72 and earlier are 4 piston.

So if that were the only 2 choices, we'd be golden, but there are all sorts of threads here showing folks how to use various disc brake set ups on A-bodies (different body lines, more modern Mopar disc set ups). I'm trying to confirm positively that's what a previous owner installed...