Based on what I've seen in the auction,I believe you have the correct buildsheet and someone changed the rearend housing & added the correct 4.10 gear.Plus the way the seller phrased it,screams the same,asing "A" & not "The".
...A 12 bolt posi rear (4.10 gears) is still in the car and documented on the build sheet to be correct.

Anyone ever figure out what the deal was with the fore/aft axle placement relative to the rear wheel well openings? Where the pictures just distorted?

That is a great question that I struggled with prior to purchase. The right side shock is frozen in a fully extended position so the car is racked badly to the left. While the car was in the shop the right rear tire went flat basically leveling out the car. Once the car was level the wheel placement looked correct. But I did want to measure it to confirm.

The seller was a Mopar guy and really had very limited GM knowledge so not sure his word phrasing was on purpose, but could had been. I know that when I scraped off years of gunk to find the axle coding it had been a long time since anyone was looking for that code. Seller told me he counted the wheel rotations to the driveshaft rotations to assume a 4.10. We are going to pull the rear end cover and see if the gears are date coded.
Will keep digging and see what this old crusty thing turns out to be....:3gears:

Here are pictures from today

More pics

More pics

Few stamp pics, clearly the D is different than the rest of the stamp but also look at the 1. And no 9? Bad day at the plant? Not sure but clearly a different stamp. I am going to soak a blue rag with evap o rust and remove the rust so the broach marks are more visible. I can see some but need a little evap o rust.

Legit stamp IMO.

Yea I agree on the stamp... if you pull the engine apart take pics of the gasket marks on the deck...

FWIW a KK rear end is WAY more valuable than a CF... double or triple, however L78 with 4.10 is brutal.