I see the carbon fiber bikes from Rocky Mountain bikes use MSL in their branding (up until 2016, at least). What for the letters stand for?
Found some info on this page
Named after its instinctive ability to tame a variety of terrain, the Instinct MSL is the perfect platform for epic exploration and now features SMOOTHWALL™ carbon construction (designated as “MSL”)
Seems that MSL is some name that doesn't really stand for anything, but means that it has SMOOTHWALL Carbon construction.