Found what I believe is a M21 trans., main case 3851325, tail 3857584 and side cover 3884685. Has one ring on input shaft and has a Hurst competition plus shifter. Has stamping on main case flange of F161017, have not been able to figure what this stamping means. Letter is definitely an F.

 

Update, found the date code number stamped by the side cover on the flange. It is P1129. but has an upside down 2 with a circle around it in between the p and the first number 1.

 

The F161017 is the VIN derivative from the vehicle that was born with that trans, with the F representing the build plant of the car.