what a fascinating antique piece! It looks like a - Fausse Montre (French) - False watch. Traditionally, these were carried on the right-hand side, in addition to the genuine watch that was carried on the left side, or carried on one end of a MACORONI (long version of a chatelaine).
The Fausse Watch did not have a time-keeping movement, but sometimes would include a small mirror, a pin-cushion, or a Vinaigrette.
Some fausse montre were made inexpensively of gilt metal while others were almost as luxurious as the real watch with a gold or silver case, having an enameled or jeweled back. They were carried by men ad women in the late 18th century.