The 6U8 (ECF82) is a high slope triode beam tetrode designed for television use as a frequency changer or mixer. It was also found to be effective as an audio phase splitter and was used in the Radford Audio MA/STA Mk2 amplifier.
The triode is on the right and the beam tetrode is on the left.
The design is clearly a beam tetrode as the presence of the bright beam plate indicates.
The thin glass tube envelope is 21 mm in diameter, and excluding the B9A base pins is 48 mm tall.
References: Data-sheet & 1040 Type 6U8 was first introduced in 1954. See also1954 adverts.