The D41 is a pair of signal diodes that share a common cathode and was designed for use in superhet receivers where the use was as both a signal detector and AGC rectifier.
The central oxide coated cathode with the small bright anodes fixed to the side rods.
The side view shows the small size of the anode/cathode gap.
The identification is etched into the dome.
The construction showing the heat marks of the spot welding of the anodes to their supports.
A close-up of one anode. It can be seen to be a single sheet folded and welded to the support. The insulated heater can be seen entering the cathode tube and at the top is two loops indicating that it passes four times through the small cathode tube. Considerable dexterity is needed to hand feed the heater into the cathode.
The B5 base
The classic envelope is 36 mm in diameter and, excluding the B5 base pins, is 78 mm tall.
References: Data-sheet Type D41 was first introduced in 1935. See also1935 adverts.