ORIGINAL: backstab
Im looking at the rx now and it has the little space on the servo plug area for Futaba. It came with a Hitec 7 CH radio. Hope that helps...
it should work then right?
Those two facts would indicate that your RCD 3500 Rx is a negative shift version and would not work with a positive shift Multiplex Tx unless that Multiplex Tx had the selectable shift software (unlikely).
The stock number of the RCD 3500 that is the positive shift version is #23672
John