The CV3930 is a special quality miniature triode for use as an oscillator at UHf frequencies up to 500 MHz.
The 5718 is the commercial Type designation. The bright metal either side of the exposed grid is the inside of the anode sheet. The outside of the anode sheet is blackened.
The cathode tube is a rectangle with rounded ends. At the top is the open edges of the anode and the edge of the grid.
The grid winding passing over the oxide coated cathode.
The thin glass tube envelope is 9 mm in diameter, and excluding the B8D/F base wires is 35 mm tall.
Reference: Data-sheet. Type CV3930 was first introduced in 1957. See also1957 adverts.