The CV5311 is a UK Government marked Mullard M8248, which itself is a special quality UHF triode for use as a grounded grid amplifier. The prototype was the VX8248.
The Mullard logo and the Type designation M8248.
The working face of the anodes are small flat rectangles backed by a larger channel. The two halves of the anode are joined by the bright strap. The control grid is close wound fine wire.
The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins, is 44 mm tall.
Reference: Data-sheet. Type CV5311 was first introduced in 1960. See also1960 adverts.