The only external  difference between a small bore ( 14.9 Hp) engine and the large bore (24 hp) engine is on the left side of the small bore  engine there is a 1/2 inch "land"  beteen all cylinders 1 and 2     and cylinders 3 and 4  (as well as a  3/4 "land between 2 and 3)  . On the large bore there is only the 3/4 inch  "land" between cylinders 2 and 3., with NO land between the other cylinders .     

The small bore engine was produced primarily for the UK and some European countries where the road tax was very unfavaourable to the larger engine.