Vacuume modulator auto trans for 75 bus

[terryconklin]

I have a VW camper from 11/74 with the auto trans. My question is about the vacuum modulator. Part #-391 for trans series NA,NB(carbs), and part #391C for series NC,ND (fule inj.) Are there any others? Are the threads the same on both modules? I have the first one, but it won"t thread into my trans. It has fine threads, and the trans has coarse threads. Jim Greene has both, and says the threads are the same on both units (fine), but Jim the bus man says they're different, so you can't install the wrong one. Does anyone know who is correct?