For 1970 Hamtramck built cars, the broadcast sheet, advance dealer shipping notice or original factory invoice to the dealership are the only places the sunroof is coded. It does not appear on the fender tag.
M as a VON prefix on early cars means the car was built as a sales bank car for some promotional use. Once that use was over, in most cases it would have been delivered to a nearby dealership and sold as a used car.
Later in the year it just means "sales bank" it is common to find the M on AAR Cuda's & Dusters later in the model year.