According to Basil Harley, author of 'Toyshop Steam'  1978, this engine was made between 1947 and 1950 by Bowman Models Ltd of Luton Beds who apparently had no connection with the pre-war firm of Bowman Models of Dereham.
There is a story that it was exhibited at one of the post-war British Industries Fairs and that a prize of £5 was offered to anyone who could explain how it worked ---
  It is not known whether this prize was ever claimed and unfortunately the Company got into difficulties. By 1950 Gamages were selling off the engines at a 50% discount.
  With the lagging and the side panels removed it can be seen that the manifolds are approx' level with the middle of the cylinders, and the passages are in fact drilled straight across/through them before plugging the ends of the drillings.

  The exhaust side of the engine with the pistons partly withdrawn shows the long exhaust 'port', which is a groove milled in the side of the piston with a passage drilled from the top of the groove up through the piston crown to the cylinder steam space.   The left hand piston groove is lined up with the exhaust port drilled in the manifold and is on its exhaust stroke, the right hand piston being turned so that it doesn't line up.  However, on the other, inlet, side of the engine where there are similar but slightly shorter grooves, also connected to the steam space, this right hand piston will have its groove lined up with the inlet port due to the grooves on opposite sides of the pistons being rather less than 180 degrees apart.
   Thus by twisting the pistons as they rise and fall the steam space in each cylinder is alternately connected to the inlet and exhaust.   This twisting is done by skewed crank-pins and the connecting rods are articulated to cope with the angular movement. Pins and shafts appear to be a press fit and the drilling and assenbling must have taken some rather ingenious jig design to keep the two ends of the shaft in alignment.