RE: Testing Receiver
I think it still could be AM.
I know I may be corrected on this, but I think the frequencies are the same for 72 mhz in AM and FM. The difference is the type of modulation. I'm thinking you have a RX on 72mhz channel 42 AM. Both AM and FM frequencies are reserved for RC use within a specified range of the 72mhz band.
If it turns out to be an AM rx, you can still use it, but I wouldn't if it were mine. I have some AM stuff, but I don't know why I keep it.