The NR88, later renamed the CV1197, is a UHF oscillator triode for use up to 600 MHz. The prototype was the RL18 that became the EC53.
The electrodes are made as a conventional valve - but very small. Both the control grid and the anode are taken to top pins.
The anode is shaped so that the working face is very close to the grid but the main surface area is well away from the heat producing centre. The insulated heater can be seen entering the cathode tube.
The NR88 is a UK Navy designation, this exhibit is one of a pair based in this oscillator from 1943.
The name plate.
The anodes are connected to the Lecher lines. These are marked in mm and the short forms a resonant stub to set the frequency.
A closer view of the electronics. There are chokes in the heater line and the cathodes are connected to ground via 100 Ω resistors (seen in red).
This exhibit is the valve in the foreground.
The thin glass tube envelope is 15 mm in diameter, and excluding the B3G base pins is 37 mm tall.
Reference: Datasheet. Type NR88 was first introduced in 1941.