I believe Global Link RV locks key their "master" keys with a "M". Otherwise, I'm not aware of any RV door locks that use an "M" in the key code.
You might get more information by calling RV Locks:
http://www.rvlocksandmore.com/Global...s#.WBkynekVCpo
This is the link to their Global Link lock page. If you can't find answers to your questions on their website, you might contact them to ask questions.