Who has the better collection? You or your mother? :D
LOL, all I could think of was ZN might mean "zinn" which might mean pewter! It's rather funny when I'm so off the mark. I had a lovely Makkum...
My specialty as a kid was getting knots out of very fine chains. There is a sense of satisfaction when it's all straight again.
I suspect John Cert made it, whoever he may be. Possibly a student or studio piece.
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What does it mean, generally, if mark is off centre @Any Jewelry?
Separate names with a comma.