The 4205E was the standard valve and the M4205E has had the grid and anode specially treated to reduce secondary emission of electrons.
The ratings say that the maximum full power frequency is 15 MHz. The short base pins are for the special bayonet base used for telephone repeaters at the time.
Thanks to Richard Stibbons for donating this valve. Richard comments - these valves were used to rectify a sniff of the transmitter output for monitoring purposes. This was at the BBC Daventry transmitter that he joined in 1962. This particular exhibit was rescued from the skip. The broken glass supports attest to rough treatment.