Gold pendant hallmark - believed to be American

by crystal
(Buckingham England)

I have a gold pendant, made I'm told by the 'lost wax method'. It was purchased in America, but has only the letters shown in the picture, these are inside the lid. It is handmade and is originally a pillbox, made from gold with hand cut garnets set in a ring round the outside.

Can anyone tell me when it was made, so that I can research it further. The 'O' has a mark inside, but we have been unable to determine its shape.