The NR50 is an acorn triode. The prototypes were the HA1, AT4 and the A40. The CV register named this Navy receiving valve the CV1171.
The AP412 stands for Admiralty Pattern 412 - the specification.
The fragile pins could easily break the glass seal when pushed into the standard base socket. Zero insertion force sockets were made but were not common. Acorn valves could not be made on conventional production lines and were considered difficult.
The five base pins - each consisting of three different metals.
The heater is tungsten as can be deduced from the connection being crimped not welded.
The original cardboard box measures 41 mm square and is 64 mm tall. Inside is a folded liner with holes for the top and base. The valve is cushioned and kept away from the sides of the box.
The acorn envelope is 13 mm in diameter and, including the base pins, is 72 mm tall.