The Z77 from M-OV was another designation for the ubiquitous EF91 of the late 1940s and here it carries the professional GEC label.
The GEC brand label. Above the top mica is a screen and also heat dissipating fins on the control grid supports.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 46 mm tall.
References: Data-sheet & 1040 Type Z77 was first introduced in 1947. See also 1947 adverts.