I agree, I know the 50020 is for the early 1990 LS400 up to date of somewhere around the 8th month that would likely put it into the 91 model year. And the 50042 runs into 92 and I think to the 8 or 9th month. The thing is everything I can think of on those years is all the same, except for front brake calipers. I know 91 at least since I have one use the same number for the ABS/Trac module, So the only possible difference I think would be a possible hardware update and programming, but all pin outs and interface to vehicle would be the same. I guess since there is no real good info on interchange of ecu's I guess I maybe the one that experiments. I need to check the caps etc before proceeding though.