Chevy Truck transmission

I have a 1969 Chevy c-10 truck. She originaly came with with a 6 cylinder motor. I'm trying to figure out what make transmission I have(muncie, borg, new process...etc). It's a 4 speed, with granny low. metal casing. All gears are very low. The tranny itself has no names on it, just numbers... they are as follows.... GM2, c 31 8, 8901131, and, 3901127. The same numbers are stamped on the top and both sides. Any help would be greatly appreciated. Does anybody know what transmission this is???
Thanks for your help, and time!!!

I think you have a SM420 four speed. I have been looking for one to do a swap into a jeep. Good transmission for heavy work!

I can't help you with the transmission But I also have a 66 GMC with a 4 speed manual transmission. Missing a Inspection cover at the base of the bell housing. Any Ideas how to get one?

[Dustin]

Here is a good site to help you:****Link removed by tjmuddslinger***** If they want the site info they can pm you