This valve is marked A3012Z and no trace of any such Type can be found. Visually it looks very much like the DET24 which could be used as an oscillator or amplifier and could produce up to 10 Watts at up 2 GHz.
The GEC paper label. This was possibly a special modification of the DET24.
The serial number on the anode end is 1210.
Right to left: heater pin, heater and cathode ring, grid ring and anode flange.
The planar envelope is 21 mm in diameter, and including the base pins is 68 mm tall.
