Why would any car be a prototype for a completely different model line? Good, and legitimate, question...Yet from all we've seen, a 383 Dart was not supposed to be built in 70. Maybe somebody changed their mind about the products for '70 and said, "Hey, maybe we shouldn't have dropped the 383 from the Dart line up. Let's try one in the new body style and see if it would work. If it does, maybe we'll release it." I don't have a problem with a car having prototype on the tag.
Have you ever seen something like that? No but that doesn't mean it doesn't exist.
Yes, someone posted a fender tag with N41 and N42, I had just never seen one and it didn't seem to make sense. Everything I have ever read said the only cars with an "M" in the VON were the AARs, although I think the T/A registry has one T/A with an "M" in its VON. While some is known about the use of SO and VON codes, many questions remain as to exactly why certain codes were used and when they were used. Some codes have a different meaning when combined with other codes or options.
M VONs can be seen in just about every body line. Below are A, B and E body tags with M codes.
A special built or prototype car would probably carry a specialty VON code of some sort.