The 2C39A is of planar design and disc seal construction and glass envelope parts. The base also includes the Type CV2516 indicating equivalence.
The dominant feature is the large external heat sink fins connected to the anode. This device would be used in a cavity as the RF Power stage of a transmitter into the low microwave region.
In operation the valve would normally be force air cooled, but water cooling is not unusual. In the latter case the heatsink assembly is held in watertight tube of copper or brass. We have such a holder in the museum.
The apex of the cone houses the heater and cathode assembly. The grid is a circular metal mesh that extends to the metal cylinder around its circumference thus providing isolation between input and output. The anode is a solid bar connected to the heatsink.
The grid: the fine wires are held taught across the support disc. This is a frame grid.
The planar envelope is 26 mm in diameter and, including the base, is 67 mm tall.