its also a way to distribute torque & power loads more evenly. if you dont use it you'll only have 4 bolts attaching the motor to the trans, with the plate you have at least another 4 tying the complete unit together.

fair call
our Nissan engineers presumably had good reason to add that plate when they brought out the later A12

Theres a good chance that I'm way off, but it was once suggested to me that the sandwhich plate is used to help loctae the starter motor. I've run cars without them, and the starter sounded terrible. (perhaps the sarter was just coozed?)

I am about to fit an EARLY 12 and a late 1979 56A 4 speed into a KB10
the early motor never used the 3 mm backing plate but the 56A came from a ute using the backing plate

so should I fit the plate or not?
also any suggestions on which bearing carrier B C or D to use?

After its regoed it will get an A15 and 5 speed and I will use the backing plate,the "D" carrier and the 60 series clutch lever but that will take engineering.
For now I just want to get the 1200 + 4 speed in and licensed.
Thanks