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Ediswan is probably producing until 1919 when production went to Osram. Fin Stewart lists MT1, MT3, MT4 and MT5 being produced by Osram (in "History of GEC and the Marconi Osram Valve", page 77.) First made by Osram lamp Works March 1919, before the MOV agreement was signed - 26/11/1919. Tube shown as made by MOV in the "Exhibit of Wireless Valves at the Exhibition of the Physical Society January 1920", page 12.
Tyne and Thrower assign it to MOV.
Similar to T-250, but for a lower filament voltage. The two tubes were interchangeable with each other, properly adjusting the voltage drop variable resistor used in series to the filament of each tube, as clearly shown in the application diagram below.
By the way, it was common practice to etch the exact voltage value on the glass of each transmitting tube with tungsten filament, individually adjusting the series resistor. (Ref. 'Thermoionic Tubes' by John Scott-Taggart, 1921)
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