Hello.
Those are Swedish marks for 1951.
http://www.925-1000.com/Fsweden_Date_Code.html" onclick="window.open(this.href);return false;
The GK mark was used by Ge-KÃ¥ Smycken, Stockholm, Sweden. Founded by G. Kaplan c.1945.
Might still be in business under the name Safir AB, Guldsmedsaffaren.
Major designers included Olle Ohisson.
Pat.