The model # 20-S is a non-production Jensen of which only 3 exist. These beautiful steam engines were given to friends and family by Tom Sr.  The 4 inch boiler with its dual immersion heaters , dual sight glasses and additional boiler port is unique to the six "S" series engines and were never used on any other models. Note the throttle valve and the extra bends in the steam line to compensate for the additional elevation of the 4 inch boiler.