The RV12P4000 is an HF pentode for use in RF and IF stages. The actual envelope is hidden within the external aluminium screen. The design follows the practice of the time of having a handle to allow replacement to be carried out 'in the field'.
The military logo.
The top pin connects to the control grid. The holder would provide additional screening for the whole length of the valve.
The handle carries the TeKaDe name and logo.
The wide metal tube envelope is 29 mm in diameter and, including the base pins, is 109 mm tall.
Reference: Datasheet. Type RV12P4000 was first introduced in 1935.