From the Isle of Man TT official page:
Grab yourself a brew, sit back and watch this enchanting video of a Rover motorcycle being constructed back in the good old days of hand crafted machinery. From casting the cylinders to machining the crank and assembling the engine, this a wonderful insight into the vintage days of building motorcycles for the road. And keep your eye out for the test rider who hits the water feature with a little bit too much enthusiasm!
The Rover company began to develop and produced the Rover Imperial motorcycle in November 1902. This was a 3.5hp diamond-framed motorcycle with the engine in the centre and ‘springer’ front forks, an improvement ahead of its time. The first Rover motorcycle had innovative features such as a spray carburettor, a bottom-bracket engine, and mechanically operated valves. Featuring a strong frame with double front down tubes and a good quality finish, over a thousand Rover motorcycles were sold in 1904.