For background see P0892.
This is a cavity magnetron developed at Birmingham for research purposes with the top cover removed to show the cavities in the copper anode block and the end of the cathode assembly. In early magnetrons the output was unstable and J. Sayers found in 1941 that linking across alternate cavities with a thin copper wire or ‘strap’ improved the stability. This magnetron has crudely inserted straps, suggesting that it is an early version.
In the image the wires through the two glass metal seals on the left (now broken) provided the high negative voltage to the cathode and also its heating current. The lead through the seal on the right provided the output, picked up from the loop in the nearest cavity.

Inscriptions / Translations:  BM9 H12 111