The VT51 battery audio output pentode was capable of 1.1 Watts at 7% distortion. The prototype was the Mazda Type Pen220A.
With the advent of the CV register in 1941 the VT (RAF Valve Transmitting) 51 became the CV1051. This exhibit also carries the old stores code of 10E/10946.
The bright box anode and above the top mica can be seen the two filament tension springs.
The top of the valve with layers of mica supports. the top support holds the filament tension springs.
The classic envelope is 38 mm in diameter and, excluding the B5 base pins, is 92 mm tall.
References: Data-sheet & 1043. Type VT51 was first introduced in 1933. See also1933 adverts.