This exhibit is a mystery, the identification is 2D1 but no such Type seems to exist. Clearly it is a full wave rectifier with mesh anodes.
The front to back dimension is 3.5 mm.
The electrodes are held in a rectangular mica as would be found in earlier balloon enveloped valves. There is a second mica that fits tightly into the top of the glass to hold the assembly rigid. Later construction would use a single mica for both tasks. The paint on the dome suggests that this once was fully screened - quite unnecessary in a rectifier.
The wire mesh anodes.
The classic envelope is 42 mm in diameter and, excluding the Ct8 base pins, is 99 mm tall.
Updated July 21, 2020. shape:classic construction:pinch type:rectifier.fw age:1930.1940 base:ct8 pins:8 pin: