How the Devil does this work?...

And so to a wee puzzle for my hoards of followers.
Anyone into steam here?  Highly skilled mechanical engineers?

A new chum has sent me pictures of this engine.  We assume it was intended to be a steam engine, but I wonder if it might not be a pneumatic engine.
Whatever its original intention, it is...a 3 cylinder, single acting, apparently valveless engine. It ticks over on a lung full of air, is immensely torquey, smooth and powerful.
Beautifully hand made, it has two eccentric sheave driven pumps, one we assume, for water and the other for oil, as it is sump lubricated (more "evidence" of it being a pneumatic?)

Here is the odd internal arrangement shewing pistons With spigots (lovely word, that)

And here is it running on a puff o' wind, sounding for all the world like a Merlin on tickover.
https://www.youtube.com/watch?v=9m44XtB08-c&feature=youtu.be

The steam/air goes in the top, the exhaust comes out a little lower.  What the horizontal pipe linking all cylinders does, we don't know.

All ideas as to what it is and how the devil it works would be very much appreciated.
It is due to go in a vintage style straight runner, for the express use of and at the Victoria Park Model Steamboat Club, the world's oldest and of which my Grandad was a founder member back in 1904.
I should also say that it is 1" bore and stroke and about 9" long in all.