transaxle identification basics

[Enrique]

I've traded for a pair of transaxles, and I get to choose from a selection of them. I have a '68 911 body that will be pushed by a '67 912 motor, so I want an early transaxle (901?). Questions:

1) Which transaxle do I want and how do I identify it? Where are the numbers and what do they mean?

2) How do I tell a 4-speed unit from a 5-speed?

3) Anything I should be looking for or guarding against?

Thanks as always.