The ubiquitous red valve, and this exhibit is clearly not in mint condition. This amplifier pentode was a general workhorse. It was extensively used in radio and radar during WWII.
Mullard, the UK arm of the Dutch multi-national Philips, describe the EF50 as being fully controlled by a grid voltage of 0 to -6 or 0 to -55 Volts, according to the circuit configuration.
The pins are equi-spaced around the circumference of the B9G pin circle. The central base spigot houses the evacuation seal as well as guiding the valve correctly into the base.
The wide glass tube envelope is 32 mm in diameter and, excluding the B9G base pins, is 60 mm tall.
References: Datasheet & 1043 et al. Type EF50 was first introduced in 1939.