Cameo Brooch on Onyx in Sterling Frame

A beautiful Shell Cameo attached to a Black Onyx background. It is set in Sterling Silver “leaf” frame and has a maker’s mark “ATK”. I have researched but unable to find the maker. It measures approximately 1.5” x 1.75” and weighs 14.03 grams. BM# 13-PGH (P-348)