I was going to say that that is one seriously obscure reference, but if you Google "1391401," you get more than 6,000 hits, and even 29 pages into the listings, most hits are about the Model M. Makes sense, I guess. There must have been hundreds of thousands of them produced, and most of those that have survived have by now gravitated to people who actually know what they are.
As I'm in your general area, I promise to honk if I ever see your plates.