A factory Slant 6 with a 8 1/4 differential

The other day my 74 duster was up on the lift in my shop because I broke my K-Frame. While I was inspecting the car for leaks I came across the differential and I notice that it was not the usual 7 1/4 diff but a 8 1/4.
I bought the car about 10 years ago I cant find any rec. of anybody puting it in before I had the car so Im thinking this was a dealer demo car?

Has anybody come across a slant six car with 8 1/4 diff in it?