The GPR3 is an AF triode designed for two volt accumulator supplies. The low filament current suggests that it is oxide coated.
The distinctive red base of PR valves carries the address of Paternoster Square in London. It is not known who made the valves but as the PR Valves address is very close to St Paul's Cathedral it is certain that this was an office location. Note that the pins are of the simple split-pin construction. The advert referenced above from 1929 shows a new address and base pins of a more modern type placing this exhibit before September 1929.
The silvering of the bulb by the gettering covers the entire surface with a mirror. This difficult to obtain image shows the cylindrical anode, the grid held on one side only by a strong strut and a single filament connection. Thus the filament is a single vertical thread and the other connection will be at the top and not visible. The filament on this exhibit is open circuit.
It appears that the lowest part of the filament is still present and held in the fold of the support wire.
The balloon envelope is 44 mm in diameter, and excluding the B4 base pins is 93 mm tall.
Reference: 1043. Type GPR3 was first introduced in 1929.