The 6K4 is a sub miniature VHF/UHF amplifier triode that Sylvania introduced in 1947 some three years before Mullard launched the equivalent EC70. The 6K4 was designed as an oscillator for use at up to 500 MHz and the design was resistant to shock and vibration.
The Sylvania logo. Inside the glass is the end wall of the anode with the grid visible top and bottom.
The five wires in the hard glass base.
The thin glass tube envelope is 9 mm in diameter, and excluding the B8D base wires is 37 mm tall.
References: Data-sheet & 1040. Type 6K4 was first introduced in 1947.