The RK-707B is an S Band reflex klystron oscillator. This would be used as a local oscillator in a Radar receiver operating at about 3 GHz. The external resonant cavity fits over the reduced width end of the valve.
Maker reference number. The repeller connects to the top pin
The Io base and further along from the pinch can be seen the cathode with heater wires entering. The emissive surface would be the flat top of the cathode can.
The discs for connection to the external resonating cavity.
The cathode end.
A RK-707B with cavity fitted.
The wide glass tube envelope is 32 mm in diameter, and excluding the IO base pins is 120 mm tall.