The 2K25 is a low power reflex klystron on a modified IO base. The output probe passes through base where pin 4 would normally be and passes into waveguide. Exhibit 2K25 is complete with housing and waveguide.
These local oscillator reflex klystrons were used for the x-band (9.xx GHz) radar as well as in industry.
The frequency can be adjusted by means of opening or closing the bellows formed by the strips at the top arm. The square end of the brass screw can be seen. The screw adjustment has a range of a few tens of MHz at 9.5 GHz. The top cap connection to the reflector is on the far right.
A diagram of the internal structure.
The thin metal tube envelope is 26 mm in diameter and, excluding the IO base pins, is 54 mm tall.