The 6H8 is a metal envelope doube diode triode and is another implementation of the final valve in the magic three series. The double diode triode acting in a superhet receiver as signal detection, AGC detection and first audio amplifier.
The Type designation is stamped into the envelope.
The top cap forms part of the vacuum seal.
The wide metal envelope is 31 mm in diameter, and excluding the IO base pins is 90 mm tall.
References: Datasheet & 3002. Type 6H8 was first introduced in 1939.