The CV1246 is a battery output triode, the previous service code was the NT82.
With an anode load of 4,500 Ohms the valve can deliver 0.3 Watts. The direct commercial equivalent is the Marconi P2, a close relative is the Mullard PM202 But not the earlier Cossor P2 which is a bright emitter.
The CV1246 has a bright box anode, and in side elevation can be seen to be small front to back.
Most CV valves lack a manufacturers mark as they are supposed to be made to a government specification by more than one supplier. This was clearly made by GEC.
The wide glass tube envelope is 29 mm in diameter and, excluding the B4 base pins, is 86 mm tall.