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 brand and Made in England. The gettering obscures the inner detail.
The original Mazda box is 28 mm square and 82 mm tall. Inside the valve was wrapped in corrugated card and an outer paper layer.
The thin glass tube envelope is 11 mm in diameter, and including the B3G base pins is 45 mm tall.
References: Data-sheet, 1040 & 1043. Type D1 was first introduced in 1937.