At first glance, this engine looks like a typical low temperature Stirling engine. However, it's unusual in that it has only one crank, which is connected to the power piston. There is no crank for the displacer. The displacer is controlled by the position of the power piston. It's not obvious how this works but I think the connection is magnetic. The displacer moves almost instantly when the power piston approaches TDC and BDC. Another unusual characteristic is that it will run in either direction.
The power cylinder is 1/2" bore x 5/16" stroke. The displacer is ~3 1/8" bore x 1/8" stroke.