Ok, I now understand your question. The M-No material designation is unique to AWS because they are the organization that develops the Standard Welding Specification Procedure and they develop their own Welding Standards (I have AWS D14.6 and materials are classified using M in this Standard). They follow the same nomenclature as ASME Section IX, however for those materials not identified by either a P-No or S-No, AWS assigns an M-No to broaden the range of materials.