Vincent Power Cycles such as this are quite rare nowadays. The Vincent Firefly was only manufactured between 1953 to 1956. In the first year it was supplied as a cycle-attachment engine kit only. When fitted to a specially made Sun bicycle, in 1954 and 1955, or this type of Phillips frame in 1956, it was known as a Vincent Power Cycle.
Vincent ceased production of their big motorcycles in 1955. So, actually, a Firefly unit fitted to this Phillips frame as a Vincent Power Cycle would have been one of Vincent’s final machines.
Image and description courtesy BuyVintage.com