Jeff,
PT with the crown is the stamp for proof testing at the French proof house in St. Etienne with French proof powder T. There was also a proof house in Paris, it used different stamps. French proof was, by law, the highest in Europe at one time. 18.4 is the metric measurement of the barrels (a 12 gauge, in this case) at a point about 9 inches from the standing breech. That is what it measured originally, it could be a bit larger today if the gun saw much use or was honed.
It looks like a very nice boxlock.
Welcome to the site.
Best,
Ted
