One of the most important developments in the history of Triumph's long-running parallel twin was the introduction of unitary construction of the engine and gearbox in 1957, commencing with the 350cc 'Twenty-One' (later 3TA). The 500s went unitary the following year but it was not until 1962 that this more modern form of construction was extended to the 650s. A new alloy cylinder head was adopted at the same time and the engine/gearbox unit went into a new single-downtube frame with bolt-on rear sub-frame. Three 650s were offered initially: the Thunderbird tourer, Trophy trail bike and Bonneville sports roadster. Benefiting from considerable recent expenditure with engineering specialists SRM, this early 'unit' Thunderbird is offered with dating certificate, Vehicle Registration Certificate and expired tax disc (March 1995). Harry says that it 'goes well'.