What would an NH or HN stamp on a gold diamond earring mean?

It does not have anything like 14k etc on it, so what is NH or HN depending on which way you look at it?

Those initials are most likely a hallmark, the stamp that identifies the manufacturer. Usually the karat purity of the gold is stamped next to the hallmark.

Gold Diamond Earring
Gold Diamond Earring
Gold Diamond Earring