This triode is very similar to the 212E family including, 248A, 308B, 4212E, ES275, MY3-275, MZ2-200, V1505 but differs in that it has a carbon anode, a giant button base without a pinch seal, a top 'pip' where the valve was pumped and is only! 295 mm high, about an inch shorter than the models listed above. The base pins and bayonet pip are identical to those of the valves listed above.These valves were used by Rediffusion in AF amplifiers to power their distributed radio system in the 60's and 70's. Both Mullard and GEC published designs for kilowatt audio amplifiers using this family of valves.
It is believed that these are late, improved version of the V1505, possibly made by Ediswan/AEI. I know of three other examples of identical valves. All appear to be unused and none has any visible markings. Possibly they were supplied unmarked. It seems closest to the 248A, but I cannot find a data sheet for that valve.
It has a directly heated cathode and a solid graphite anode capable of dissipating 275 W. It has a special giant 4-pin bayonet base. Normal operating conditions were an anode voltage of 2 kV and current of 180 mA, giving ra of 2000 ohms, gm of 7.2 mA/V and a µ of 15. Here is full Mullard data, from "Valves for Industry and Communication" part 2, 1954, for the MZ2-200, in .pdf format.