The D77 is the Marconi equivalent of the Mullard EB91, and consists of two separate signal diodes in the envelope. The QD77 is a special quality version of this valve.
The diodes were designed for vision and sound detection in television receivers. Additionally they are recommended for limiting and DC restoration.
The logo is for Marconi Wireless Telegraphy.
One diode with the screen between it and the second diode.
Typical diode construction of a small central active area and a wide flange for fixing and heat dissipation.
The thin glass tube envelope is 17 mm in diameter and, excluding the B7G base pins, is 47 mm tall.
References: Data-sheet & 1040. Type D77 was first introduced in 1949. See also1949 adverts.
