The two on top  cylinders is normaly used on British Cyclemasters. 

Left: 25cc cylinder with 32mm bore. Cooling fin diameter is about 79 mm.

Right: 32cc cylinder with 36 mm bore. Cooling fin diameter are here
about 87 mm. You can see what cylinder your engine is equipment with by measuring
the cooling fins without dismantle the cylinder head.

Lowest:32cc cylinder used on german engines. The cylinder has sloping rear cooling fins