The Series 1 was introduced, initially for export only, in March 1961. The domestic market launch came four months later in July 1961. The cars at this time used the triple SU carburetted 3.8-litre six-cylinder Jaguar XK6 engine from the XK150S. Earlier built cars utilised external bonnet latches which required a tool to open and had a flat floor design. These cars are rare and more valuable. After that, the floors were dished to provide more leg room and the twin bonnet latches moved to inside the car. The 3.8-litre engine was increased to 4.2 litres (4,235 cc) in October 1964.