The 394A is a mercury vapour thyratron designed to be used as a controlled rectifier.
The anode is a flat plate at the top that is fixed only to the top of the envelope. Inside the screen is the helical filament and control electrode. Near the pinch is a mesh mercury reservoir.
A clearer view of the mesh.
Paper label from the end flap of the box.
The rest of the label attached to the side of the plain corrugated card box.
The classic envelope is 45 mm in diameter, and excluding the IO base pins is 128 mm tall.
