Mystery mark on 10k Art Nouveau style brooch

Does anyone recognize the stamp on the back of this little 10k Art Nouveau style brooch? I am not versed in jewelry - not sure of the age which is why I said 'style'. The mark looks like a book or box? Overall the piece is almost an inch. Not stamped but tested for gold purity. Not worth much money but just curious. Thank you!

 

First, I highly doubt it's gold. It's made all wrong for precious metal and dead bang for costume. Art Nouveau Style is correct; I'd guess it's more 1990s than 1890s. The clasp mechanism is all wrong for earlier, and the while rivets were a 1950s method, it just doesn't look right and wasn't used on gold ever as far as I know. The gold on the back doesn't look like real gold; the texture looks like older gold-filled items would.

 

I'm seeing an O and a G as sides of indeed a box shape