The CV121 is a directly heated high voltage half wave rectifier. The prototype was the V1920.
The pressed metal anode is darkened to radiate more heat and so allow the valve to run cooler. The construction is rugged and unusual.
The picture of the anode end of the valve shows the ceramic insulator and also the substantial strengthening ring. Also seen are the spiral filament tensioning springs for the two inverted V filaments.
At the base end the pinch is topped off with a ceramic insulator and support. Fixings are also aided with stout wire rings. The filament supports are also clearly seen as is the detain in the anode. This latter electrode is ribbed for enhanced rigidity. This rectifier appears to have been designed to withstand rough handling. Possibly in airborne or shipbourne Radar sets.
The wide glass tube envelope is 48 mm in diameter and, excluding the B4 base pins, is 175 mm tall.