This valve is classified as part of the NR series of pre WWII Naval Receiving valves.
This valve is equivalent to the commercial AC6/Pen and was designed for timebase output in early TVs and oscilloscopes. The cathode resistor is given as 90 Ω.
The Type designation uses the - as if it was an American NR-74 but the AM and crown confirm that it is the UK NR74 for Navy Receiving.