Wow, what a fascinating brooch! I know I’m late to the party here, but it looks to date from about 1850-1890, based on the construction of the pinback hinge and catch. Looks like there’s a number on the back – 850, maybe? If so, it may be a silver purity stamp, stating the parts per 1000 of silver in the alloy. (Sterling silver is 925/1000, 92.5% silver.)
It says SILBER 0.990, so I was at least confident about the silver part. Thanks for the information about the hinge and catch. I picked it up at a street sale in NYC over 30 years ago and have loved it ever since. Just figured I’d try and find out something about it now that answers are usually just a click of the mouse away.