Gearbox identification

Hi all,
i have a spare gearbox i'm going to open and overhaul (if i can afford it!).

i just take a look to the repair manual and there is several types of gearbox actually HERE THE LINK
RN4F30A
RS5F30A
RS5F31A

RS5F32A
and

RS5F50A

the only thing i can find on my gearbox is the number stamped on the top of it : 3906 ACR

Does any one here know how to identify ours (or mine if it is different...) or tell me where i can find something about that?

edit : Only RS5F31A and RS5F50A are 4WD and i suppose roughly that the F50A are ours.

Edit 2 : i confirm that the F50A is ours.