A good start would be to determine if the nutplates are SAE or metric threads...
OR if they are British (BSF or BA) threads.
That will narrow the field considerably.
With metric threads you will need to research if different European countries used different thread pitches (if you are not familiar with thread forms ask someone with engineering knowledge or ask here and someone will provide a link to thread basics.
I know several different European countries used different spanner sizes for the same diameter metric bolts so it is possible that they also used different thread pitches. I do not see any bolt heads or plain nuts but accurately measuring them with a micrometer or digital vernier would be a great start. If rusty then wire brush them to remove as much scale as possible and measure as many faces as possible to get as accurate an average as possible. Brit and US head sizes are far different and metric has at least two options.
By far your best clue would be finding a part number or inspection stamp on a small part. If you find a circle with an H and a T inside it that is the heat treat stamp used on Brit and US aircraft. I do not know any of the European heat treat stamps but they are probably different and that alone may provide a clue as to the country of origin (most of the languages are different though some are very similar) as I am sure someone here knows what those countries used. (If someone here does know this would be a good place to widen everyone else's knowledge)
A stamp with AC or AN, usually in a circle indicates a US aircraft. An AID stamp indicates British. Again I do not know the Russian, German and other countries equivalents and again others will. (Again if someone knows please enlighten us who are not as knowledgeable).
Part numbers can be a world of knowledge or a world of pain. Part numbers starting 75 can be Boeing, Curtiss, North American or Stearman - and that is just the US manufacturers that
I know used that prefix. There could be others. Plus Brit and Euro options.
From the stringer shape it is almost certainly not Curtiss or Bell but it is a common stringer profile used by many.
The short skins are similar to used on the Me-108/109 but the frame shape is wrong for the 108 for certain and if my rusty memory is correct it is also wrong for the 109.
Don't give up - this is probably the best site to crack the puzzle.