Thinking Bf109 engine bearer bolt - but that doesn't fit with what I think was a clue. But I can't see anything to suggest maybe a mg locking bolt either .......
Not a P-61, not WW2, not a float plane, and not Japanese !
Oh, an it's only got one engine.
Oh, and as it's possible to find two distinct types of this particular aircraft, I need the exact type to be identified.
Yes, I know, I'm a bar steward !