How early the cavity magnetron might be developed

How early could the cavity magnetron plausibly have been invented?

The 1920s are the earliest plausible window, but only if German research circulated sooner and received sustained funding instead of being neglected until the mid-1930s.

The earliest plausible development falls in the 1920s, not because the physics was unknowable but because the research path was fragmented. Earlier circulation of German work and a stronger institutional push would be necessary to turn proposals into a practical device.