Welcome to ModelEngineMaker !If you have problems registering or logging in, please use the contact menu option to request assistance.
The angle of the valve chest is to keep the valve rods axis in line with crankshaft. Any other arrangement would need some kind of crank lever mechanism to have equal valve motion. But why a 15 degree angle? Why not 23 degrees or 12 degrees? Again it might be space. A shallower angle like 10 degrees would require that the crankshaft be further from the cylinder block while a steeper angle like 20 degrees would move the crankshaft and the cylinders closer together. That, however could cause interference between the cranks and the cylinder rod end. And it is easier to manufacture to a common angle than to something like a 23 degree slope, so 15 degrees.
One of Stew's photos shows what looks to be a steel braided hose attached probably with a quick connector, as to whether it has steam or air I can't say. There is a small handwheel on the vertical shaft coming out of the top of the valve presumably to control flow into the valve chest.Possible air run for demos in the mill as there may no longer be a steam supply plus it saves a lot of red tape.