When Operator said about Stanley, It rang a bell, so checked the yesteryearstools Collins history and he has this tid-bit "That liquidation, in June of 1966, resulted in the four out-of-country manufactories being purchased by The Stanley Works of New Britain, Connecticut."
On Ross' Woodtrecker history it has several names the later axes were sold under, none of those start with "M". It could be that one of those outside the USA factories purchased by Stanley is the Mexico M maker. So it would appear on Collins, then Stanley, and some of those names on Woodtrekker history, of course depending on when the name was used. What do ya'all think?
I still plan to check the heat treat on those M heads. Not sure how soon, long work hours this week