Seems there were a lot of manufactures that made the parts over the years. I dont have an answer.
I’ve read a number of books, including “The Black Rifle”, on the subject, and none of them gave any credibility to a single Mattel story.
Sometimes, I watch Discovery and History and even The Military Channel, and constantly slap my palm to my fore head (AGNTSA).
Even saw a very smart, educated and factually proficient professional claim on The History Channel that in Vietnam, the M-16 rounds would “swell in the chamber overnight” and lock up the action. I’m sure he meant something else, but his exact words were bizarro.