The CV1091 is the service version of the EF50. The CV valves normally had no obvious makers identification, the red paint of this valve suggests a Mullard product.
The valve is a high impedance pentode and was a workhorse of receivers for both radio and especially radar.
The wide glass tube envelope is 32 mm in diameter, and excluding the B9G base pins, is 57 mm tall.
References: Data-sheet & 1040.