The VX8121 is the STC development valve that was designed to meet the Ministry of Aviation specification CV4501.
The design use for VX8121 is not clear but the specification gives the requirements for the noise factor at 24 to 50 MHz. Typical Radar IF frequencies. Noise performance is significant for the initial stages of a receiver.
The base is a B8D/F to denote the flying leads.
Copper supports and a blackened anode for heat dissipation. Significance of the coloured paint at the top?
Half of the two sided anode with central strap. The bright plate is connected to the suppressor grid.
The thin glass tube envelope is 9 mm in diameter, and excluding the base wires is 38 mm tall.