The 12E1 is a power output valve for use as a series pass valve in stabilised power supplies. The heavy demands placed on valves used as control elements in regulator circuits makes it vital that the ratings are not exceeded.
The classic envelope is 50 mm in diameter, and excluding the IO base pins, is 123 mm tall.
References: Data-sheet & 1040. Type 12E1 was first introduced in 1952. See also 1952 adverts.