GEC was the trade name of M-OV valves destined for the professional equipment market. The Type is given as EF91/Z77 as the EF91 was the generic name for this valve type.
It is a high gain, high impedance, high slope, screened pentode, primarily intended for use as RF amplifiers or mixer valves in VHF television receivers.
The GEC label.
The thin glass tube envelope is 17 mm in diameter and, excluding the B7G base pins, is 46 mm tall.
References: Datasheet & 1040 Type EF91 was first introduced in 1947.