The N339 is a purpose designed TV line output valve from the era of 405 line VHF monochrome television receivers.
The peak anode voltage can rise to 7,500 Volts and it has a 12 Watt anode dissipation and a 4.5 Watt screen dissipation.
The reverse is also labelled and the anode prevents the grids from being seen.
The thin glass tube envelope is 20 mm in diameter, and excluding the B9A base pins is 70 mm tall.
References: Datasheet, 3002 & 1040. Type N339 was first introduced in 1952.