The ST has been made in several versions since 1923. This is a later version dating from at least 1959. I thgought, at first, that it was an earlier version because it has no cylinder cover. However, the rest of the engine is as per the later version so I assume that the cover is missing.
It is a single cylinder, double acting, oscillating cylinder marine type engine made of bronze. The cylinder is 7/16" bore x 7/16" stroke. The crankshaft has a disk crank and a 1" diameter flywheel all steel.
The engine appears to have been built from a set of unmachined parts and, as acquired, was not working. It required straightening of the trunnion pivot and removal of a rough spot on one of the port faces. After lapping of the port faces and running in, it now runs ok.