I came across a diffrential,The case reads FA 15 67. the
Gear set was GM 4.56, The confusing part is the ring gear was on a series 3 carrier with a spacer plate.I don't think the General would have pushed it out the door that way.My guess is that it was dealer or some previous owner installed it. I dont even know if 4.56 was a production option. And if it were a series 4 carrier would have been used.Anyone ot there have a clue?

My numbers book says FA is a 427 with a 3:70 gear.