The 6LU8 is a Compactron triode pentode designed for television receivers. The medium-mu triode was designated for use as a frame oscillator and the pentode as the vertical deflection amplifier. The 6LU8 was designed for parallel heater systems and the otherwise electrically identical 16LU8A was intended for equipment using series heater chains, usually domestic TV receivers.
Although referred to as a triode pentode the construction is as a triode beam tetrode.
The beam tetrode face view. Unlike the 16LU8A the control grids are not on copper support rods.
The central screen, connected to the BP and thus the cathode of the beam tetrode, can be seen to have ridges pressed into the metal to give rigidity.
The dome carries the identification. On the left is the beam tetrode with the additional mica supporting the heater.
The envelope is 38 mm in diameter, and excluding the B12C base pins is 67 mm tall.
Reference: Datasheet. Type 6LU8 was first introduced in 1967.