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 large canopy is a screen, the two anodes are within. The lower anode is visible, but the top anode is hidden under the screen.
The etched lettering on the dome. The Type designation is at the top and difficult to read.
The wire at the top is the cathode connection. The Type is also marked on the pinch.
The balloon envelope is 41 mm in diameter and, excluding the B5 base pins, is 73 mm tall.
References: Data-sheet & 1040. Type DD620 was first introduced in 1934.