Phil,
The date code indicates it is a late 64, probably installed in a 65 vehicle. L - Dec, 10th day, 64 - year.
64-65 transmissions were kind of odd as they had a different pitch on the gears compared to earlier units.
My understanding is that GM did this to get rid of the Chevy transmission whine we all got used to. It was a two year configuration prior to the introduction of the full syncro Saganaw which was available in 66. There is nothing wrong with these transmissions, get the electrical components operational and it should work fine.
Fred
