The 6KA8 triode sharp cut-off pentode was designed for the American 525 line television market. The design purpose was as a gated AGC amplifier and noise inverter. The feature of the valve highlighted in the data-sheet is the dual control characteristic.
The RCA logo. The triode is on the right and the screen is the bright line.
The triode is on the left and the pentode on the right. Note how the suppressor grid is close wound, this allows it to act as a second control grid.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 59 mm tall.
Reference: Datasheet. Type 6KA8 was first introduced in 1962.