This is a prime example of a Goose Neck steam passage. These are what happens when a slide or “D” valve is used and they are all bad. The “Goose Neck” steam passage is long and wasteful. It is clearance space, so it has to be filled with steam every time, a dead loss for that steam. Then when exhaust comes through it the metal is cooled off and that needs to be re-heated with incoming steam. The Mobile Steam Society people say that these kinds of valves, which are what Stanley uses, are the best thing invented to turn steam into water.
Stanley 20 hp cylinder sawn in two. Generally these old Stanley blocks have the valve face all spalled and galled from running with too hot a steam and too little cylinder lubricating oil. I am assuming that was the case with this one. Coburn Benson had this one prepared so that one could see the steam passages. These are technically referred to as ‘Goose Neck Passages’ and they are very long thus making the engine very inefficient. On the other hand it was a simple engine design and easy to make with the type of casting and machining technology of the time.