This is a Sanwa cover For Sanwa RX-471/RX-472 receiver, where you can put your MRT PTX transponder in.
The hole for the antenna cable is to be found on the backside.
How to:
Open the receiver, glue the transponder into the cover and solder it to the power supply of the receiver (free slot).
If you do not want to solder, take the cover for AMB RC4 transponder.