The 3Q5GT is a dry battery audio output pentode with an output of, typically, a quarter of a Watt. The filament has a centre tap so that it can be used with 1.4 Volts or 2.8 Volts, i.e. one or two dry cells.
The reverse has an etched octagon with the Type designation within. It is possible that Ferranti bought this from RCA.
The oval anode.
The two filament strands are held in tension springs that hold the filament tight to the edge of the top mica.
The wide glass tube envelope is 29 mm in diameter, and excluding the IO base pins is 58 mm tall.
References: Data-sheet, 1040, 1043 & 3002. Type 3Q5GT was first introduced in 1938. See also1938 adverts.