The 6AG7 was designed for single ended audio output stages. Most likely use being for radio receivers.
The make and origin are printed on the back of the envelope.
The thin metal tube envelope is 25 mm in diameter and, excluding the IO base pins, is 68 mm tall.
References: Datasheet, 3002, 1043 & 1040. Type 6AG7 was first introduced in 1939.