The 11D5 is a double diode triode with a 150 mA heater designed for series heater chains in domestic superhet radio receivers.
The Brimar name on the base cap. Also the fact that it was made in England.
The design follows the standard format with the triode at the top - grid to top cap and below is a screen within which two rods act as the diode anodes. The diodes being used for signal detection and AGC rectification. The bright box screen connects to the cathode.
A closer view.
The classic envelope is 38 mm in diameter, and excluding the B7 base pins is 96 mm tall.
References: Data-sheet, 1040 & 1043. Type 11D5 was first introduced in 1939. See also1939 adverts.