The 35L6GT glass tubular audio output valve was designed for the final stages of radio receivers. The 35 Volt heater 0.15 Amp heater rating was to enable the valve to operate in series heater chains in universal sets, including television receivers. The 50L6GT is identical in operation but has a different heater rating. It too was designed for heater chain use.
The coating makes a clear view difficult but the anode is a rectangular box with three ribs pressed into the sheet to confer strength and help maintain its shape.
The silver gettering shows off the etched identification.
The wide glass tube envelope is 27 mm in diameter,and excluding the IO base pins, is 65 mm tall.
References: Data-sheet, 1040 & 1043. Type 35L6GT was first introduced in 1938.