 The first numeric character indicated the filament/heater power rating, the second alphabetic character was a code for the function, and the last 2 digits were sequentially assigned, beginning with 21

RMA tubes

RETMA is the acronym for the "Radio Electronic Television Manufacturers Association" formed in 1953.Often designations that differed only in their initial numerals would be identical except for heater characteristics.For examples see below

A four-digit system was maintained by the EIA for special industrial, military and professional vacuum and gas-filled tubes, and all sorts of other devices requiring to be sealed off against the external atmosphere.