Here is a comparison with the 1870- alphabet frame, I don't find a match with that one. Maybe another 1870 frame? It seems to be made with high craftsmanship!
for my research on it i reached a dead end to .
But revealed a few things nonetheless
1 there are jewelers who have trade markings in diamond shape ,
2 but no German jeweler so far ..
3 frame inner corners ,close to the DOV frame but not the same ...sadly
So trade mark as diamond shape as such are possible ,,
the secondary stamp is not a full N or Z as far I have seen.
unknown to me so far .
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