This piece of vintage fire fighting equipment was manufactured by the Rockwood Sprinkler Company in 1942. It is called the Waterfog. It has three settings, Off, Fog, and Open. In fog mode, it takes in air through the small hole in the front. This mixes with the water and creates a fog effect.
Built of brass, with a nice patina. We bought it in Charleston, SC, and were told it came from a ship. We've seen ads for Rockwood products that show similar units being used on US Navy ships, so it's certainly possible. Very cool item.