What is the difference between all the A body gas caps????

Yeah, it do: the "1971" cap is really '70 California + early '71 50-state/Canada, and late '71 through '76 caps are interchangeable, and '60-'66 A-body caps are a group unto themselves.
X2. Here in Ca. there was an early '70 cap (before 1/1/70) and a late '70 cap which was like the '71.