Two pole voltage indicator with neon lamp inside.
Type B, Patent No. 830153, Factory Inspectorate Certificate No. I.S.3177
Made by T.S.G Seward, 11 Station Road, Stockton-On-Tees.
The internal components can be seen clearly, consisting of a resistor in one probe and a resistor and neon in the other.
Both probes have a crack along the length of the tube, possibly due to the resin inside expanding.
The instruction sheet is shown as a medium sized image. A full size 600dpi version can be downloaded here. (PNG image, 224KB).