The SP41/U from Cosmos carries the Metrovick logo as well as the Shortpath branding that was used up to 1928. Under the etched type designation on the envelope is the word (half) in parentheses. The design dates from 1927 and the literature states that it is directly heated. Observation of the electrodes shows an oxide coated cathode tube and an insulated heater. Three connections are taken to the base along with the anode. Clearly an indirectly valve but probably with poor heater/cathode insulation given the date of the design.
Physically the valve is large for the modest current rating.
This side view shows the getter holding plate as well as the Shortpath construction, where the anode is a flattened open sided cylinder to reduce the distance from cathode to anode. At the right can be seen the top fixing for the heater with the hairpin bend and the top support held in a glass insulator.
Another view of the un-silvered side. Note the BVA logo on the left.
The BVA logo with the words British Radio Valve Manufacturers Association.
The base end of the electrodes. The wire seen at the top of the picture is the support for the getter. The cathode tube can be seen to be formed from sheet that has been rolled into a tube.
The balloon envelope is 60 mm in diameter, and excluding the B4 base pins is 120 mm tall.
Reference: 1043. Type SP41/U was first introduced in 1927.