South American Pinfire Cartridges are pretty uncommon! Pinfire guns had a relatively significant use in South American countries such as Argentina and Uruguay as many were imported for many decades. The large 15mm pinfire guns were especially popular by farmers as a large side arm to protect from wild animals.
This is a high-end 15mm pistol that came out of Uruguay. It was made by the luxury gun manufacturer, P. Boissy somewhere around the late 1850s to 1860s. He manufactured his guns in Saint-Étienne, France and even won an award at the Exposition Universelle of 1855 in Paris for his “pistolets de luxe.”