sentences:  According to Jeffry L'H.Tank's "History of the Motorcycle", Iver Johnson advertised their machines as "Mechanical Perfection," a boast that was not entirely unbelievable given the number of advanced design features in especially their later models, such as dual crankshafts, nickel-alloy machined parts, chain drive, and a hand-operated three-speed gearbox.Among collectors and researchers who have the benefit of hindsight, Iver-Johnsons, such as the 1915 Model 15-7 along with Scotts, are the finest examples of motorcycle engineering of that era.