This valve has been in the museum for some years but could not be identified. Recently (2015) a Frelat valve was sold on ebay and looked like the mystery valve. Close re-examination showed feint indications of the Frelat logo as well as the words Made in Holland.
Frelat valves were made by M Heussen & Co of Arnhem in the Netherlands and exported to the UK between 1926 and 1928. This time frame would indicate a dull emitter valve rather than the 'bright' of the R Type and so the Type designation may be incorrect.
The simple split pins set in a Paxolin disc.
The Frelat valve sold on ebay showing the style of the name logo.
The balloon envelope is 50 mm in diameter, and excluding the B4 base pins is 77 mm tall.
Reference: Observation & 1047. Type R Type was first introduced in 1916.