The EF50 was the ubiquitous red valve for a decade, and here we have a Mullard EF50 in a plain aluminium can.
The EF50 was designed in 1938 by Philips of Eindhoven for Band I television receiver use and first released in 1939. It was extensively used in radio and radar during WWII. One additional reason that so many existed was that they had a short life and so were changed often.
The can is stamped with two BBC valve section test stamps and shows that the BBC had this valve in 1951.
The wide glass tube envelope is 33 mm in diameter and, excluding the B9G base pins, is 59 mm tall.
References: Datasheet & 1040. Type EF50 was first introduced in 1939.