I agree that they have an M1 confusion problem going on here. You're right, the M1 should be reserved for the supercar heritage. But I'm guessing there are going to try to "unify" the M numbering with the series number, and this WILL be the M1.
What SHOULD they call the ///M version of the 1 series? Some of my suggestions are serious, others are jokes!!
///MHatchback
///MCompact
///MPoseur
///MRicer