The PR3 is an audio triode sold by Peter Russell valves. He claimed British manufacture for his valves but the actual maker is unknown.
The envelope is fully silvered due to the gettering. The base has a vertical line to denote the anode pin.
An identifying characteristic of PR valves is the orange/brown base that always has the words - PR Valves and the address of Paternoster Square in London. Note that the base pins are of simple split brass construction.
Looking onto the dome the etched lettering is in poor condition.
After a lot of experimenting details of the inner construction started to be seen.
The electrode assembly in relation to the envelope dome
Finally this image was obtained. The glass bead is held by struts to the anode (right side) and grid (left side) supports. The centre pin in the bead carries the two filament tension springs. Two springs indicate a pair of inverted filament strands.
Re-processed from the previous image, this shows one filament strand.
The balloon envelope is 45 mm in diameter, and excluding the B4 base pins is 97 mm tall.
Reference: 1043. Type PR3 was first introduced in 1928. See also1928 adverts.