NP207/231/242
I think some came with another one, but the 231 and the 242 were the most popular. Both are about the same, strength-wise, both chain-driven. The 242 has a 2.72:1 low range, and I THINK the 231 does too, but theres a 4:1 tera lo kit for the 231.
Try and stick with the 231 and 242, they're better cases. If you wanna do swaps and whatnot, the pre-91s were 21 spline input, and post 91, they were 23 spline input. They could be either if it's a 91.
Oh, and there are a lot more aftermarket products for the 231 (SYE, 4:1 kits, etc).