The D1 is an all glass signal diode. It was especially designed with very low inter-electrode capacitances to function as a video detector for VHF television. This would have been a signal of up to 3 MHz bandwidth. These characteristics made it a natural choice for RADAR systems where diode detection was employed.
The shaped anode plates leave a tubular hole for the cathode assembly. The anode plate fins clamp round support rods for stability. Mica top and bottom plates complete the construction.
The Mazda identification and Made in England.
The insulated heater entering the cathode tube.
The anode in full face view. The connection from the anode to the top pin can be seen.
As well as 3 pin bases these small signal diodes were also held in plastic housings similar to fuse holders. The valve would push down into the base clips and then moved horizontally to engage the anode. A direct vertical push on all connections could break the glass to metal seals..
Looking down onto the valve in the holder.
The thin glass tube envelope is 11 mm in diameter, and excluding the B3G base pins is 35 mm tall.
References: Datasheet, 1040 & 1043. Type D1 was first introduced in 1937.