who knows? - somehow I think that runic alphabets and latin into greek or vice versa is perhaps reading a bit too much into this symbol - looking at related acid etched marks for a similar time period for Leerdam - shown in your 'nl/abel/leerdam' notes - I'm not getting the impression those were created using cryptic borrowings from other alphabets............... just my opinion of course.
My knowledge of Leerdam symbols and marks is virtually non-existent, but surprised to hear that a symbol on a piece from such a well-known and documented output should remain a mystery, still.
But hey, if it wasn't a mystery, you'd have nothing to worry your mind with

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