I based my original identification on your statement in post #1 that the dimensions did not match 7.92x57mm ("...though dimensions are not exactly as those I found on the web") and the fact that it had an FN headstamp.
As the case length does in fact match the 7.92x57mm then I stand corrected and that is what it almost certainly is. The fact that it was made by FN is still in keeping, as FN made millions of rounds of 7.92 on contract for many countries.
Regards
TonyE
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