Mystery bellhousing, need ID

My neighbor picked up a saginaw 4-speed for his '65 C-10 project to replace the granny-gear trans in it now. It came with a bellhousing I've never seen before. It appears to have two sets of block patterns, one for the Chevy and the other set I'm ***uming is Buick/Olds/Pontiac with provisions for the starter on both sides.
What would this thing have come in originally?

 

I believe that it may come from a mid to late 70'sgm car. They were jambing BOP and chevy engines into any gm car at this time, some bright engineer may have had the idea to make a universal bell housing to simplify the ***embely line

 

I've seen a couple of those that came from early '80s F bodies.

 

That makes sense as the trans has the bosses on the tailshaft for a torque arm.