That is a really interesting piston; thanks for uploading the photographs.

I am not sure exactly what it is but I am sure what it isn’t (and I agree with almost everything that has been said so far: not Junkers or Daimler-Benz).

It certainly comes from an engine with (only) two valves per cylinder so that rules-out about 95% of Battle-of-Britain crash sites (if it is indeed from one of these sites). It is also most probably from a radial, as opposed to a V-12 type, engine. (The two valves also rule-out any of the British sleeve-valve engines, incidentally.)

Some features are distinctly American in nature but that doesn’t rule-out it being German. As I’m sure you are aware, some German radial engines of the period owed quite a lot to earlier American radial engines (as do some Russian and some Japanese radial engines).

I have a photograph somewhere of a broken Bramo-Fafnir piston that was claimed to be from a Do17 crash-site; I will see if I can find it but it looks nothing like this piston?