From the same time frame as the Swan and Cygnet...
is this Stuart Turner 800, not a model but a small full sized gas engine of 1/4 HP.
There are very few of these out there.
This one was purchased as raw castings by Andrew Burr ca 1958 when he was 18 years old. A neighbor on Long Island, Harry Goodman of the Long Island Live Steamers built this one for him as well as a Stuart 5A. Harry was a highly skilled machinists and builder and did a wonderful job with this piece.
It is a hit & miss engine but rather than hold the exhaust valve open, the governor lifts a tab to stop activating the fuel injector until it needs to fire again.
In order to bring this home I had to take on 26 engines.... Andy Burr's entire model engine collection. The other 25 were all steam engines.
Enjoy!
Gil