Kontax are one of the better makers of quality Stirling engines.
This is a fairly conventional low temperature Stirling engine, except that it's made to a high quality standard. It's claimed to be able to run off the heat from a hand, although I've found this is only so if the air temperature is low enough. It will run on my naked thigh!
It has miniature ball bearings in the crank shaft and a graphite power piston in a glass cylinder. The power cylinder is 9.5mm bore x 8mm stroke. The displacement piston is 70mm diameter x 7mm stroke.
Condition is used but almost as good as new. This example comes with the optional glass dome cover.