Over the years, I've always heard/read about "M" denoting Mach 1 stripes so the line worker doesn't have to go looking for 63C instead.
There's charts of what paint color gets what stripe colors, but for a customer order that deviated from the norm, I have no idea where that is denoted on the build sheet.
A build sheet decoder would be great to have in the resource section.
It's weird how info on them seems so scant, it's not like it's classified LOL!