Found a 66 D100 yesterday at pic-a-part that was picked clean other than the rear end so, I figured I'd grab it for my 63 D100 if I ever wanted to get rid of the tapered axles. Plus it looked to have 3.55s by counting turns, which is what I was going to throw in for a lil better freeway rpm. Has 3.91s now and plenty of low end with the Poly 318 and NP420 4spd. Weird thing is the casting number on the junk yard drop out is 1634983. With a little web searching, it appears to be an earlier (1957??) open drop out someone threw in it. I read there may be different thrust block thicknesses between some earlier and later units but, axle end play seems fine. Not high priority now but, I'll pull it all down and check it out further. Just kinda curious what pinion/bearing configuration it is. Figured I'd post it as I have never seen anything other than 741, 742 or 489 on 8.75s...

1634985 is the fairly common earlier version of the 742, the 983 appears from time-to-time but not sure it's a large stem pinion or not.

If the flange axle end play is OK, it's either a Sure-Grip or the open carrier has the wide thrust block.

'985 is equivalent to the later '742...983 is equivalent to the later '741

Then what's the difference between the 1634983 and 1820657? The 657 is also equivalent to the later 741.

There were also several other casting numbers used.