Found a truck for sale online that says it has an Allison 645 6-speed auto in it. But I tried google and nothing came up, and the shifter only has R, N, 2-5, 2-4, 2, and 1 on it. I'm thinking it's probably a 545?
It's an IHC with a 466, built 4/89, if that makes any difference.

 

545 non lock up 4 spd. Descent combo.

 

What I suspected, thanks

 

It could also be a 654. That is way better than the little 545.