This exhibit carries the printed details for Ferranti but also on the glass are the etched details that look very much like an RCA product. The 50L6GT is identical in operation, and has the same etched details, but has a different heater rating.
The 35L6GT glass tubular audio output valve was designed for the final stages of radio receivers. The 35 Volt heater 0.15Amp heater rating was to enable the valve to operate in series heater chains in universal sets, including television receivers.
The etched details - probably by RCA.
The only visible details are above the pinch. The control grid has copper supports.
The wide glass tube envelope is 28 mm in diameter,and excluding the IO base pins, is 72 mm tall.
References: Datasheet, 1040 & 1043. Type 35L6GT was first introduced in 1938.