"Twin Power Turbo," on the engine cover means twin scroll. N55 is a twin scroll SINGLE turbo, not twin turbo. Made from MY 2011 - 2013. N54 is the twin turbo engine. Made from MY 2008 - 2010. These years are for the 135i.
Here is the list of US convertible 135is and Canadian convertible M Sport Edition cars.
http://www.1addicts.com/forums/showp...&postcount=329