Earliest possible FAA cannon armed fighter

With what POD?

I think a single 2pdr on a twin engined flying boat for attacking surface targets might work in 20s?
Becker M2 was a 20mm proto-oerlikon the Germans had fitted to a variety of aircraft by the end of the war. Aside from the hundreds handed over to the allies at wars end, SEMAG of Switzerland were selling licensed copies in the early twenties (before merging into Oerlikon who put their name on the design). If the Germans could fit them to an Albatros in WW1 the Brits could fit one into an Avro Bison or Blackburn Dart in 1924, and the FAA has cannon armed aircraft from the day it is formed. How useful a box-mag 20mm would be is another matter, maybe ‘Dirigible Interceptors’ or something?

Aircraft autocannon were a WW1 innovation, no need to dawdle about until the late thirties if you really want them. But if you don’t want them enough to spend money developing them yourself or rustle up a quick licensing agreement, then you end up where the brits did.
 
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