That could mean anything. It's a code intended for the folks who build the car, not so much as an indicator for the customer. My guess is the M represents a color code. The '1' for some other option that's not so obvious. (leather or vinyl trim? wood trim? cut outs in trim parts for optional switches, etc...)
I'd guess that label is used by build folks and inspectors in the assembly plant build process to ensure the car is built to perfection.
And cars.com trim levels may not be the same codes the assembly plant uses... Its totally possible that there are three different level of interior plastic trim parts, used on six different marketing 'trim' levels as displayed on the cars.com site.