This marker generator, made in 1980, was designed to produce a comb of frequency markers up to and including the 10 GHz band.
The diode used seems to be a receiver point contact type. The design would have been a crystal oscillator and buffer amplifier feeding 48 or 96 MHz to the diode. The non linear action of the diode would then generate harmonics up to 10 GHz.
The top view shows the waveguide clamping block and the diode mounting near the closed end of the waveguide 16.
Seen from the side the feedthrough capacitors taking the DC voltage to the crystal oscillator are visible.