OK Alex, then the number '4' is out of the question, as inscriptions/marks are seen from the hilt, and this would mean that the number '4' would stand on its head - which is very unlikely.
I really don't know what it is, but should I guess, and I mean guess, then I would say it is a makers mark, or possible an armoury mark - but why on both sides of the blade?
Jens
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