According to what I found it is a Chysler product for the most part. startin in the6\mid 60's on up. Even used as an overdrive for some vehicles. had 2 different spline feeds but the next paragragh seems to fit what you have found since you show the bellhousing from a Chevrolet
(GM used this same transmission with GM's own 10-spline input shaft, input bearing and retainer:
o 1981 to 1986 Chevrolet 2- and 4- wheel drive pickups and Suburban (3.09:1 ratio first gear)
o General Engines designated this transmission code MY6.)
Good luck
