Any 7.9x57 ammo would probably work in a BESA, but it wouldn't be possible to tell whether they were used in one or not. Unless it had a British military headstamp, in which you could safely say that it was for the BESA as the UK didn't use that ammo in anything else.
However, none of the headstamps you show are British. FN is obviously Belgian, and FS is probably Fabrique Solothurn of Switzerland.
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