Markus sent a few pics of the timing chests of his engines, showing the differences between the Model 18 engine (top) and the Model 21 engine (below). Note the "IT" racing exhaust cam in the M21 engine; the exhaust cam of his M18 engine is marked "K" and I would not know the significance of that stamping.
Markus: "In 2005 I had a bad experience with the Model 21 engine; I was overtaking a scooter at about 100 km/h when the engine suddenly lost power! You can imagine how embarassing that was...So I stopped at the side of the road and found all compression was gone. I pushed the Norton home for 3 kilometers at 30°C, opened the engine, and found a huge hole in the piston...I could see directly into the crankcase! Perhaps the combination of an old (non-Norton) piston and wrong ignition timing was the reason for this failure. I've now fitted specially prepared drop-forget Mahle pistons in both engines and haven't experienced problems since."