All Mini models have R-series model numbers assigned to them. These are a legacy of the Mini's original development within Rover Group.
R50: "Mk I" Mini One and Cooper (2001-2006)
R52: "Mk I" Mini Convertible (2005-2008)
R53: "Mk I" Mini Cooper S (2001-2006)
R55: "Mk II" Mini Clubman, S and D (2008-present)
R56: "Mk II" Mini One, Cooper, S and D (2007-present)