As far as depth originally they were just stamped by hand, and depth changes from letter to letter and so on, Also stampes were changed out as time went on, I started to look into these more after pulling one from my 68, Mine looked to me as the gears and posi were changed out in 1970 my carrier is a HD unit and gm 4:88 gears all dated early 70 Mine I think is real even with a early date it was just in a old race car that hadnt seen service in over 30 years, and as me for a long time racers really do not care if its rare or not. I also have another 4 searies carrier (empty) that came with my car that is damages looks like a pinion nut had came off and drove the pinion into it. I think alot has to do with where a part came from to decide, most of that things life it was just a Camaro 12 bolt... here are some sample pics
