By Chance, did you get this one at the gun show in Maumee OH? I saw this exact Luger (or one that is incredibly similar) there in late March and had decided to pass it up since it had no visible toggle stamp or other proof marks beyond that "hk" in a circle on the receiver. Otherwise I too look forward to more answers on this topic since I was researching it the rest of the day and could find nothing for these markings.
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-QM
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