The black coating you are seeing is the dip coating that the body was immersed in, the rocker blackout and inner fender blackout was applied later and is a different in that it's paint and not dip sealer/primer, and the shade of black is different.

When I steam cleaned my 8.75 rear end I not only found the "A" but MANY other markings and ink stamp details that were invisible before I removed some of the grease and rust. Mine looked similar to yours, a fine rust patina all over it which hid a lot of details, most of that wipes right off with a rag and water. If you clean it don't just have someone sand blast it, etc. go nice and slow using the mildest cleaners possible gradually progressing to more aggressive stuff so that you don't accidentally errase some hidden inspection marking details.