The Plessey GDHM32 was the Gunn oscillator module of choice for wide-band 24 GHz equipment in the late 1980's.
As with all microwave sources a box in a box arrangement provided essential stability. The Gunn oscillator is seen here built into a die-cast box with waveguide as the mounting point, the fixing screws pass through holes in the die-case housing outside of the actual oscillator cavity. In this case the GDHM32 was used as a test oscillator. See also G2SDP's 24 GHz wide-band transceiver.