The Jefferson motorcycle was the brainchild of Perry E. Mack, who débuted his new vertical-overhead-valve engine in 1910; one of the earliest motorcycles to have valves 'upstairs'. Sold initially as an engine only, under the Waverly name, the motor soon gained a reputation as an excellent racing device, undoubtedly due to the better breathing afforded by overhead valves.
The company reorganized in 1911 and relocated to Jefferson, Wisconsin. The marque was re-named the 'P.E.M' to honor Perry Mack. The motorcycle industry was cutthroat at that time, and small manufacturers, even if legendary on the racetrack, came and went in a matter of years.
In 1913, the name was changed again, to the Jefferson, after the town of manufacture, and a V-twin engine machine was introduced, also overhead-valve, which also proved a fast, well-built, and reliable machine for road or racing.
An interesting feature of the roadsters was the use of short-link suspension both front and rear; movement in each case was controlled by short bellcranks connected to leaf springs, providing perhaps 1.5" movement fore and aft to absorb bumps; another innovation from the Waverly company. Innovation and race wins simply weren't enough to keep small manufacturers alive in those brutal early days of the American motorcycle industry, and in 1914 Jefferson ceased trading. There is at least one surviving example of the Jefferson.
Interesting fact: The Jefferson motorcycle was only made for one year--1913!