M0 has different meanings for the 1970 model year.

Early cars like yours it means Show car / Sales Bank.

It would have been used for some type of promotional purpose, car show, magazine road test, print ad or commercial to name a few possibilities. Once the purpose was met, it would have been sold at a nearby dealership in most cases.

Sunroof cars in '70 were normally noted by a K in the VON, but in some cases like this one the K got the boot & the M0 was used due to the car's intended purpose - whatever that was.

Later in the model year M0 just meant Sales Bank (Y16) as found on most AAR Cudas & some other models like Valiants & Dusters.