The Z77 from M-OV was another designation for the ubiquitous EF91 of the late 1940s and early 1950s.
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.
See also Z77 for a Marconiphone branded valve.
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.