Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

This is a typical design for high speed single acting steam engines.
Most of these were used in model boats and some from Stuart Turner
were used in small field generators by the military.

Stuart made a lot of different models with the same basic design.
Jim I suspect that as stout as that is that it was purpose built for a good sized model tether boat.
Tether boat engines are mostly single or twin cylinder however H.E. Boucher made 4 cylinder engines as did a few other makers. 

High speed tether boats operate at up to 1,000 psi.