The Z77 from M-OV was another designation for the ubiquitous EF91 of the late 1940s. In this exhibit the lettering proclaims a commercial brand of Marconi products. See also Z77 with a paper label.
The design offers high gain, high impedance, high slope, in a screened pentode.
The electrodes fill the width of the valve. The three grids can be clearly seen and all grids have separate pins.
The thin glass tube envelope is 17 mm in diameter and, excluding the B7G base pins, is 46 mm tall.
References: Datasheet & 1040 Type Z77 was first introduced in 1947.