Hi James
The RM stands for record maker.
Most of these did not have the additional record maker parts fitted so there should be an 9 pin valve style base fitted to the socket.
https://www.radiomuseum.org/r/pye_fenman_ii_rm.html
The differences between the RG and RM are tiny.
I think I published a snip about the difference eons ago on this forum.
crossed with Edward
Cheers
Mike T