Saginaw or Borg Warner?

[strick9c10]

I have the opportunity to purchase a parts truck for my '79 C-10 project. I haven't seen the truck in person yet, although the owner has snapped a few pictures & sent over email. It has a 4 speed in it & he says the transmission has an RPO code of MM4. Does this code signify a Saginaw or a Borg Warner? It's the original unit behind the original 350 4bbl. The parts truck is also a 1979.

I've researched on line & can't find any code info... only that it's a 4 speed. I have info on visual identification, but I'm not there to verify.

Any help would be great. Thanks!!