I have to agree with Quarter Scot. Pull the center stud and all four barrels fire. However, rather than fore and aft, I think it was designed for port and starboard. If one had a man holding each arm."Sorry Officers. Let me show you some ID."

In reality I think Mr. Macdonald is more correct. The lock features (as near as I can see in the still photo) a well developed gooseneck cock, a flat plate and roller frizzen spring. All were generally later 18C design features in British locks (but much earlier there than this side of the pond).