M-OV who made the Marconi Z77, acknowledged the fact that this valve was universally known by the Mullard designation of EF91 and gave the EF91 top billing on the envelope.
The EF91 is a high gain, high impedance, high slope, screened pentode. Mullard describe the EF91 as primarily intended for use as RF amplifiers or mixer valves in television receivers. At the time, late 1940s, television only operated at VHF.
It could be used from AF to VHF up to about 200 MHz.
The electrodes fill the width of the valve. The anode is formed by two rectangular plates at either side of the valve. The other two sides of the box being completely open. All three grids can be clearly seen.
The thin glass tube envelope is 17 mm in diameter and, excluding the B7G base pins, is 46 mm tall.
References: Data-sheet & 1040 Type EF91 was first introduced in 1947.