Achilles,
They aren't trans part numbers. The trans part number (usually under caked on dirt/garbage) is found (as you are looking at the engine) on a rib on the right (US passenger) hand side, facing the ground and just forward of the gear carrier/trans case line. They all start with "B" and another letter.
So the differences. Pre mid '70 ish vs later.
Early GB's had the trans drain plug, 2 temp switches, temp selector switch and exposed spring seal.
They then changed to no drain plug, single temp sender, no temp selector switch and internal spring (red) seal.
Good enough as a general guide.
My '70 donor bug has an early trans, my '71 super has a later trans.
Since you have three (lying around?), go to the tech section and look at the topic Bruce has posted regarding low gear. You may care to have a look at your trans'. Also, post up your trans codes and I'll find the table I posted up about what the trans code means.
Enjoy
Volkenstein