STC a subsidiary of the American Western Electric company issued valves with a 4 prefix to the American Type designation for UK made valves. Some versions of the Wecovalve had coloured spots to indicate their best use. No colour spot can be seen on this exhibit.
This 4215A is fitted with a British B4 base whereas the Wecovalve had a miniature pea-nut base that fitted into a small bayonet base.
The Wecovalve name with details of British patents below.
The electrodes of this miniature valve are held in glass beads. The anode can be seen to be a flat sheet that has been rolled into an open cylinder. The grid is helical and the filament is a single strand. The tension spring can be seen at the lower left of the image.
The thin glass tube envelope is 15 mm in diameter, and excluding the B4 base pins is 73 mm tall.
References: 1043. Type 4215A was first introduced in 1923.