The 6U8 is a triode pentode from 1954. 6U8 is the American designation and ECF82 would be the Mullard type designation if the two were exactly identical. In ECF82 Mullard put both designations on the envelope indicating equivalence.
The design purpose was in VHF radio and television between 80 MHz to 220 MHz. Bands II and III. The valve would be used as a mixer with the triode as local oscillator.
The triode section with additional surface area to the anode on the left.
The pentode section can be seen to contain bright beam plates and is thus constructed as a beam tetrode. At full size the screen grid wire can be seen.
The thin glass tube envelope is 20 mm in diameter, and excluding the B9A base pins is 47 mm tall.
References: Datasheet & 1040 Type 6U8 was first introduced in 1954.