I'm none too sure what this is! Pretty sure it's Stuart, it's a large compound launch engine. There are a pair of cylinder standards, I would have expected two more. There are no cylinder blocks, but there are end covers, showing the large LP and smaller HP bore size, valve chests with Stuart covers, four well-machined eccentrics, slide valves, heavy seven-inch diameter marine flywheel and a large collection of sundry bearings, eccntric straps and miscellaneous castings.
2nd October - One of my resident Stuart experts has been on the telephone to identify this as a Stuart 6A launch engine. There were only two cylinder standards, the cylinder block bolted atop with pillars down to the bedplate like the No.1. A large engine, rated 4 bhp at 750 rpm on 120 psi steam. Capable of driving an 18 to 22 foot boat. Drawings and castings still available from Stuarts - the full set of castings is currently listed at 1470.
Full specification and a picture of the completed engine on Stuart's 6A page