This valve has very faint etched writing on the envelope. After many hours of looking at the envelope obliquely with a glass the words Special and Radio Micro were made out. This valve has a thoriated tungsten dull emitter filament and is of French origin. The design dates to 1923 or 4. It is believed that Radio Micro was a brand name of Sociiti La Radiotechnique. The company changed its trading name to Dario in 1927.
The special refers to the thoriated tungsten filament.
The horizontal cylindrical anode, single thread filament and wire helical grid points to the early form of French hard valve construction.
The filament is a very fine wire. The grid is welded to the support rod at each turn.
The far end of the filament is unclear but shows that the filament is central within the grid.
The electrodes above the pinch.
The wide glass tube envelope is 38 mm in diameter, and excluding the B4 base pins is 80 mm tall.
Reference: Observation & Andy Cowley. Type Special was first introduced in 1923. See also1923 adverts.