A pair of signal diodes are to be found in the DD620, A Mazda valve from 1934, intended for use in universal mains sets (transformerless for AC or DC use). For this application the heater cathode insulation has been designed to withstand a high voltage difference.
The dome carries the identification.
Within the screen that is connected to the cathode are a pair of small anodes for the diodes. The top anode connects to the right side support and the lower anode connects to the rod on the left.
The wire from the cathode goes to the side supports, the screen and ultimately to the pinch and base pin.
The balloon envelope is 41 mm in diameter and, excluding the B5 base pins, is 81 mm tall.
References: Datasheet & 1040. Type DD620 was first introduced in 1934.