The 3GK5 is a shielded grid triode. The data-sheet refers to the neutrode circuit which is a type of neutralised arrangement where a single triode of this type can replace the cascode input stage that was previously required. This type of valve was originally known in America as a guided grid valve.
The basic design concept is that shield plates around the grid and between it and the anode reduces anode to grid capacitance and allows higher amplification at UHF.
The remains of the paint giving the brand name.
The end of the electrodes.
The anode us shaped so that only a small flat section comes close to the grid - this helps reduce the a-g capacitance.
The typical internal construction of guided grid valves. Image courtesy of thevalvepage.com
The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins is 47 mm tall.
Reference: Datasheet. Type 3GK5 was first introduced in 1961.