Cracked it, I think...

OK, nobody grabbed my chance to explain the odd engine in the previous post.
But we don't care about that, do we?  That ain't going to stop me having a go and, whilst observing how revolting the average Mum is at the Grandchildrens' school, it came to me.

Firstly, I was never convinced it was a steam engine. I became more and more certain it was, or was intended to be, an air engine.
With that arrogant assumption firmly in place, I realised that the very reason it was a three cylinder was that it HAD to be to work at all.  A proper three cylinder engine is a 120 degree crank, so that there's always a power stroke going on somewhere.
Secondly it is a cyclical engine.
Thirdly, it is valveless, or at least as we know it, Jim.

The long spigot that pokes out of the piston crown serves in the office of a valve and this is how:-  after getting a nice charge of air, the first piston in the cycle is on its way, first down, then back up the bore. This would normally mean that the higher it gets, the more compression it would cause, thereby sapping power, but just before this happens, the spigot plugs the inlet port allowing the compression to be sent out of the small hole in the cylinder head next to the inlet port, which is the pipe to the next cylinder.  That cylinder is now getting its full charge from the air supply too, thanks to the 120 degree crank.  It is therefore a compression assisted following cylinder, which will do the same as the first and on to the third, whereupon the whole cycle repeats itself.
The presence of an oil pump is clearly because such a machine with brass pistons and cylinders would need some direct lubrication.  The suggestive presence of a war pump is merely that somebody decided this baby could run as a steamer.  I think it could, but I think with so much pipery it will suffer from a lot of condensation. It will also need a lubricator per cylinder because the fine content of oil in a normal steam engine would all be lost with so much transfer of vapour going on.  That aspect is open to conjecture, sure.

As a slowish speed engine in a straight runner it could be a perfectly good steamer.  What it isn't, I am certain, is a high speed steam engine. All that brass would not tolerate the temperatures and pressures of a flash steam plant.

So, what we have is a valveless, compression assisted Uniflow engine, designed for air, workable with steam.......and I claim my five pound voucher.