UPDATE: Our friend BV just sent me this, which explains the stamp in detail (thanks!!!!):
If PP hasn't let you know, the engine pictured on the stamp is a Otto Langen Free Piston Atmospheric Engine, circa 1867. A small flame, like a pilot light on a stove was at the base of the cylinder. I do not know the starting procedure, but as the engine was running, if the wheel started to slow down, a gas, like natural or coal gas, was admitted into the bottom of the cylinder, and a little trap door would open next to the base. The flame would be drawn into the base, igniting the gas, and drive the piston upward. This would push the vertical rack gear attached to the piston and engage the gear on the flywheel shaft, increasing the speed of the wheel. The inertial energy would be stored in the rotating wheel, which drove a load attached to it. As the flywheel slowed down to a certain speed, a governing mechanism would start the process over again to keep the engine running until the fuel supply was removed. You can see one at the Rough & Tumble Museum at Kinzer, PA. It is run for exhibition days.