The 50L6GT is an audio output beam tetrode and is designed for series heater chain use with a heater current of 150 mA. Essentially the design is modified from the 6V6GT and designed for the same application within broadcast radio receivers.
The main image shows the etched Type designation within an octagon border - American made.
The 50B5 is a B7G version of this design with lower output power.
The Ferranti brand is overprinted on this imported valve.
The box anode delineated by the break in the internal coating.
The wide glass tube envelope is 28 mm in diameter, and excluding the IO base pins is 73 mm tall.
References: Data-sheet & 1040. Type 50L6GT was first introduced in 1941. See also1941 adverts.