[Arran]

Dan;
  A 6F7M is electrically the same as a 6F7 but has an octal base instead so using a 6F7 with an adapter should work fine, unless the tube is being used as a mixer-oscillator, then the extra lead lengths could cause a problem, but I'm pretty sure that they were used as IF amplifier tubes or detectors, not oscillators in this set. There was also an RCA tube called a 6P7G that is the same as a 6F7M but has a different pinout so it could be used as a sub. This assumes that you actually need a 6F7M with it's triode-pentode, but what I noticed in my Rogers 10-12 is that they had the triode sections in each tube connected as diodes with the grid and plate tied together, they may have done the same in the D.F Buckingham. Since they used a pair of 6F7Ms, right next to each other, you have a number of options, use adapters for a pair of 6F7s, use adapters for a pair of 6P7Gs, rewire the sockets to accept 6P7Gs, or rewire one socket to accept a 6B8/6B8G, and the other socket to accept a 6K7/6K7G. In my case I will probably rewire the set to accept a 6B8 and a 6K7 since 6P7Gs were never that popular to start with.
Regards
Arran