Frank Wesson, (the brother of D.B. Wesson of Smith & Wesson fame) designed it, developed it, and described it in an advertising circular as "a double shot pocket pistol with extension dirk knife." It was available in three calibers: .41, .32, and .22 -- all rimfire.
The Wesson was a boxy little over and under pocket gun. The bottom barrel was deployed by taking the pistol in both hands – one hand on the grip, the other on the rectangular barrel housing – and twisting, moving the top barrel to the bottom and bringing the bottom barrel up and under the hammer. A neat way of giving the little pistol two shots, and – not incidentally – making it more accurate. Oddly, a tiny dagger was inlaid between barrels on the right side of the frame, to be slid out and locked with the push of a finger. Jack had never seen the tactical need for that particular feature, but he thought it was pretty bully nonetheless. A bantam .41 with a retractable, four-inch bayonet. He laid one of the Wessons on the bed and broke open the other. The hinged barrel housing swung down and he took two rounds of .41 Rimfire from the bed and loaded it. It closed with a snap, and he laid it down and loaded its twin.
Both derringers lay on the bed, fully loaded. His eyes traveled the black padding. What else?"