This is a four piston engine with only two cylinders.  One piston in each cylinder drives the crankshaft directly.  The other piston is cross-linked to the crankshaft on the opposite side.
Steam is admitted via an oscillating valve on the top of the cylinder block.  The steam pushes the two pistons apart, thus driving the crank.  The ends of the cylinders are not sealed since the two pistons act as moving seals.