This exhibit is a French Dario Resistron triode. The maker, type and 'Made in France' are very feintly etched into the envelope. The English spelling suggests a valve for the UK market.
Dario made the Resistron in both Fourvolt and Bivolt filament ratings and we have examples of both.
The Resistron name indicates a valve made in the late 1920s when transformer coupling between stages was being replaced by resistance/capacitance coupling. Increased gain of the valves made this possible.
The standard B4 base has the connections marked on it. The pins are simple split pins.
The filament is a single strand of thoriated tungsten wire passing centrally through the grid spiral. When energised it glows bright orange. The grid wire is held in place by being welded on one side to a support rod.
The wide glass tube envelope is 38 mm in diameter, and excluding the B4 base pins is 78 mm tall.
Reference: 1043. Type Resistron Forvolt was first introduced in 1928.