This double diode triode could have been designed as a signal plus AGC detector and first amplifier in superhet receivers. The 10LD11 is the same valve but with a 15 Volt 100 mA heater for use with series heater chain.
The B8A base with equi-spaces short hard pins and central locating spigot. The side pip on the skirt provides location information and slides into a slot on the skirt of the base.
Half of the envelope is taken up with the getter holder.
The thin glass tube envelope is 20 mm in diameter and, excluding the B8A base pins, is 53 mm tall.
References: Data-sheet & 1040. Type 6LD20 was first introduced in 1950. See also1950 adverts.