The Fotos TM was the original hard triode produced in France during WWI. When demand outstripped their capacity another lamp manufacturer, Compagnie Generale des Lamps, became involved, their trade mark was Metal and the valve TM Metal. There are differences in the construction that can help to distinguish the different makes. In particular the Fotos spherical envelope touches the base whereas the Metal made valves have a short stem before the spherical part of the glass as can be seen in the main image.
The grid helix has each loop fixed to a support wire. Here the thin grid support can be seen being held in folds of the two arms of the twisted supports emerging from the pinch.
A closer view.
The single strand tungsten filament is held, like the grid support, in folds of the outer supports.
Looking into the anode at the grid helix.
The brass base cap, four simple split pin connections screwed into the ceramic base with the copper lead-out wires soldered at the top.
The balloon envelope is 55 mm in diameter, and excluding the B4 base pins is 100 mm tall.
References: Data, 1047 & 1003. Type TM-Metal was first introduced in 1916. See also1916 adverts.
