M does not stand for Miyata, search the forum there is a detailed discussion of Miyata serial numbers. Not aware of anyone using a designation to match brand name, but there is probably a brand out there that does. Wrap around stays and dropouts are both good signs, lack of chrome on stays not as good, although there were plenty of high end models without chrome. Steering tube will usually be marked with at least tubing mfr. Tange uses a simple date code.
Tmar has a wealth of info on serial numbers.