The Triotron S201A was released in 1927 as a general purpose triode with a 2 Volt 0.1 A filament. The construction of this valve looks as if it is an earlier design.
One alternative possibility is that the oxide filament was not reliable and manufacture reverted to the tried and tested thoriated tungsten filament. Another possibility is that it was released for use as a replacement in older sets.
The identification sticker.
The box anode with grid within.
Here the grid can be seen as a pressed metal cage where the vertical components are flat and pointed at the anode. Within can be seen the filament - it is thoriated tungsten and on test glows bright orange. The S201A will have an oxide coated filament.
The balloon envelope is 54 mm in diameter and, excluding the B4 base pins, is 111 mm tall.
References: Data-sheet. Type S201B was first introduced in 1927. See also1927 adverts.
