Ok I have had few people ask me about 75mhz modules. Ok back in the day when RC car racing used 75mhz modules and transmitters they were set in a way to allow any 75mhz channel crystal to work in the same module. Later as RC racing got bigger and more people complained about needing to open the module case to swap 75mhz crystal, so something had to be done. As during racing some people would need to change the crystal channel so there wouldn't be 2 people on the same feq during the race. So later Futaba and other RC companies that made 75mhz modules made a hole in the side of the module to allow fast swapping of crystals when needed for race events. That is why is new later modules you see the hole in the side of the module. Also some people are confused about 75mhz Dual conversion and Single conversion crystals. Dual conversion and Single conversion crystals only apply to the receiver and not the module TX crystal.