I'm sure you are all right regarding the RJX EFIS. I asked a guy at Woodford who told me it was the same fit as our ex-Thai -300s, however he probably didn't realise the fit we had. It makes a lot more sense to have the tapes. Also, I have an idea that it has something to do with the digital data bus on the RJ/RJX, which I gather is not the same as the 146 has.
Mind you, ANYTHING would be an improvement on the old 146...