I think that the m designation was an indicator to Armourers and Quartermasters that the gun was of Canadaorigin and might come with certain parts or more likely, a combination of parts with which they are not familiar. It was made known in the equipment regulations that regardless of this, that where a PART might not be interchangeable, ALL the main sub assemblies are fully interchangeable. A couple of parts that I do recall were barrel nuts/latches and spring and bipods