Many early motorcycles were really motorized bicycles, a normal bicycle with some small engine either adapted or specifically designed to attach to the bike for power. This particular bit of engineering is the Bugatti T72, a 12.66cc single cylinder, twin cam supercharged engine. Think about that.
The bicycle the engine is attached to in the above photo is a 1937 Peugeot. The engine appears to be specifically built for bicycle use with mounting points designed as part of the engine case. The T72 was a little screamer that would rev to 10,000 rpm! It had gear driven overhead cams and a supercharger. If you like neat early engineering, this is a great example.
These photos and information come from The Bugatti Revue, a web site put together by Jaap Horst of the Netherlands and there are a lot more photos you might want to check out. This site was pointed out to me by Sean, one of our readers from New Zealand. Thanks, Sean!
Link: The Bugatti Revue