The Radio Micro Special 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 60 mA filament.
The base with the ribbed lower section is characteristic of Radio Micro and Dario. The pins are of the simple split pin type. The triode is mounted horizontally.
Looking into the silvered dome. The filament is a single strand and the three electrodes are mounted concentrically. Each turn of the grid is fixed to the single support rod.
The geometry of the three electrodes. The anode is six milimeters in diameter.
The wide glass tube envelope is 38 mm in diameter, and excluding the B4 base pins is 79 mm tall.
Reference: Observation & Andy Cowley. Type Special was first introduced in 1923.