This 1296 MHz parametric amplifier is a special piece of equipment. In the days when receiver front end valve and semiconductor devices had high values for noise figure the parametric amplifier could offer lower noise figures for receive pre-amplifiers.
Utilizing a varactor diode as a capacitor whose reactance is varied periodically at a high microwave frequency, known as the pump frequency, this non-linear parameter change is used to couple energy to the signal. This can by via a negative resistance effect at the input frequency or by mixing to a product frequency. In any case mixing products exist and are known as idler frequencies. Each of these idler frequencies has to be terminated.
The normal pump source would be an X-band small signal Klystron such as the 723A/B.
The top of the protruding cavity is a length of waveguide 16 closed at the outer end.