The NR47 is the UK Navy version of the classic directly heated 25 Watt anode dissipation audio power triode PX25 designed by M-OV.
The inside of the envelope is blackened. The filament is intact but at 4.0 V and 2.6 Amps gets hot but does not glow. Also at 6.0V it still does not glow but the envelope gets too hot to hold.
The tops of the filament loops with tension springs held in a glass rod that is itself fixed to the anode struts.
In close-up the formation of the tension springs can be seen. Unlike NR47, where the spring is formed into the hook for the filament, here the spring tip is folded and holds an S shaped wire hook. It is not clear how the hook attaches to the spring but it is probably spot welded.
The balloon envelope is 65 mm in diameter, and excluding the B4 base pins is 140 mm tall.