It would have to be an early car, off the top of my head I do know of at least one M VON Challenger with a sunroof.

A good case in point is the AAR Cuda. Most AARs were Y16 - sales bank cars & they have the M. But, no sunroof available on any 1970 Cuda.

Early show cars were built as Y16 cars that were to be sold as used cars after their intended show use was over with. In the case of early Challenger sunroof cars, the M trumped the K in the VON.

Very early M cars - Y16 Sales bank / show car

Later M cars - Y16 - sales bank.