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I found one similar to yours. By your middle finger on the back is a pin?
If so I think it has something to do with a decorative belt to a lady's dress.
That's just a guess. Hopefully you get a definite answer.
The button is most certainly a Tombac button, c. 1790s. The other piece is most certainly a ladies sash buckle of some sort, given the bar on the back and the "male/female" construction of two halves that fit together. You have the "female piece."
Regards,
Buck