Where (if anywhere) on the 70-72 A bodies does it actually designate the body group?

 

In the VIN and on the cowl tag. The series number tells you what it is. For example, 31xx-48xx series cars are A-bodies. 52xx-68xx series cars are B-bodies. 84xx-86xx are C-bodies. 94xx-96xx are E-bodies. This varies by year and division, so don't assume this applies to every year.