The D41 is a pair of signal diodes that share a common cathode and have the specific function of signal detection and AGC rectification in radio receivers.
The above picture shows the construction. The heater is folded so that it passes through the cathode cylinder four times. The anodes are made from sheet and enclose the cathode for a part of its length.
This valve was at one time in the hands of the BBC as the sticker shows.
The classic envelope is 34 mm in diameter and, excluding the B5 base pins, is 75 mm tall.
References: Datasheet. Type D41 was first introduced in 1935.