I also don't show a "M" engine code in any Ford or Merc manuals I have. As a matter of fact the cammer isn't covered in any service manual I own, which covers most FOMOCO products from 63-68 in my library. There have been a bunch of crap coming out if the woodwork lately. Did anyone see the Dark Pony Shelby supposedly built all while engulfed GT350 Program? I used to go over and talk to Little Joe and Carroll Shelby at the Gardena Tire Distributorship. Shelby was never a warm fuzzy hugger, as this person claims. He was brash, to the point and often countered anyone who had an opinion that differed from him. Usually with colorful language. What I appreciated is he wasn't a person who said things behind your back, he was confrontational and direct.
I know most people get butt-hurt over people who are direct these days. It's why the cry-babies have an issue with Trump. He doesn't candy-coat and is a doer, not a talker. He follows his vision. I agree as a former military officer that sometimes diplomacy is over-rated and simply waste time and resources.
Anyway, back on topic: Without Ford documents that prove without a doubt this was built by Ford it just becomes another cool car.
-Keith