Antique Sterling Silver Scottish Penannular Tara Brooch Agate Connemara Gemstone

Antique Sterling Silver Scottish Penannular Tara Brooch. Set with Agate and Connemara gemstones. To the back there is the normal as to be expected'C' clasp. But next to it there is an open oblong piece - no idea how this works! To the back of the piece there is the owners. Scratched name L M Peet. Diameter approx 4.25cm x 6cm. Please check out all my other items.