The front half (the part towards the front of the tractor) is an Oliver part, and is the same on all of them, 1600 on up to 2-105. I just went through the transfer case on my 1655, and the parts book showed the front half as a 107 737A, with an alternate of 166 511A, which should mean the second one is different, but compatible. The 166 551A was used on the 2-105, so any of them should work.

The 16xx sized transfer case had a different back half and gear that goes on the pinion shaft.

I'm going to guess that the snap ring came off the pinion shaft and let the gear grind against the front cover?