Based on photo post by Carling in 2009 query, the late-mark "GSC" for Stern Brothers from 1913 to cessation. Five-point Star, formed by main design, at outer top center. Size 10. Word marks "Gold" & "Silver" above and below of unspecified standard, but might we expect 10K and Sterling based on their other work? Is this clearly gold and silver, or gold-plated silver?
Might as well add the link on "gold filled": https://en.m.wikipedia.org/wiki/Gold-filled_jewelry Is it possible that after 1913 this firm was still marking wares in a manner that could pre-date the regulations of 1906?-on concerning filled or rolled productions?