The CV408 (commercial A1714) is a planar triode and therefore designed for high frequency use. The prototype was the VX3052.
The main use was probably VHF television at 45 MHz, which is the same as WWII Radar IF frequencies.
The anode dissipation is 2.5 Watts and running an RF amplifier at a moderate current was a way to keep the noise low and the CV408 had a noise equivalent resistance of 550 Ohms. At 45 MHz the input resistance was given as 40,000 Ohms. The maximum anode voltage was 250 Volts.
This second picture has the support micas on each side and the main electrodes lie in from front to back.
The electrode assembly lies horizontally within the glass envelope. within the outer mounting frame appear to be two small cylinders separated by the grid, as one would expect in planar construction.
A close-up of the electrode assembly.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 46 mm tall.