I learned C1 a year ago. According to Callerlab's Basic Challenge (C-1) Definitions (10/12/06), the definition of the "Left" modifier begins "Do the call interchanging right with left, clockwise with counter-clockwise, and promenade with reverse promenade." I'd heard of "Reverse" and "Mirror" and even "Arky"—none of them as yet official modifiers. I'd supposed that "Reverse" applied to "Beau" and "Belle" and "Arky" to "Men" and "Ladies" (to follow Callerlab's antique terminology), while "Mirror" perhaps combined "Left" and "Reverse".
Do the call interchanging right with left and if also applicable belle with beau. (Left should only be used to modify calls where a right hand turn or shoulder pass is clearly part of the definition. The term Reverse is commonly used when the major interchange is Beau with Belle such as Reverse Swap Around.)