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.

This early 'unit' Thunderbird if offered with green continuation logbook (issued 1971) and old/current Swansea V5/V5C documents.