12 bolt rear question

[OldChevy61]

When I started building my 62, I ditched the ho32 and replaced with a 12 bolt. Casting no,s came back as 63,64, 3,73, non posi. It is a 6 lug. My son just bought a 66 c/10, has a 12 bolt ,3.73,non posi. Two totaly different differential covers. His is clearly truck, mine has the cover of a car rearend. When I bought it, was out of the vehicle so no way to know what it actually came out of. Whatever it was had trailing arms and panhard bar, measured the same as my 62 rearend, Just had to move the mounting pads and panhard bracket. Was there any Cars in that time that used a 6 lug rear? It works real good , just curious as to what it may have came out of or if the earlier ones were different.