The Douglas Experimental Department at the Kingswood factory BS15 1920.

"Experimentation and innovation drove development right into the First World War. The new sport of motorcycle racing was a powerful incentive to produce tough, fast, reliable machines. These enhancements soon found their way to the public’s machines. By 1914, motorcycles were no longer bicycles with engines: they had their own technologies, although many still maintained bicycle elements like seats and suspension."

Via: brizzle born and bred's photostream