Rover EFI identification

[en]

I have a 1992 ECU and EFI harness i'd like to use with an 1988 intake plenum.
I'm ordering throttle shaft bushings and TPS for the '88 plenum.
The parts guy is asking me about 31D or 38D vs
36D or 40D
I'm not familar with these terms.
Can someone point me in the direction of this info?
Also, is the 3.5 TPS different from the 3.9 TPS?

Thank you

[tr8todd]

Those correspond to engine numbers if I'm not mistaken.  They are stamped into the block just below the head on a protruding platform.