I cannot imagine why they would put 10% silver in a coin... except maybe to be able to "legitimately" be able to market them as "silver coins" for some reason. Or maybe it was something like certain WWII era US nickels where there was some silver (35%?) used in place of nickel which was in short supply for weapons and other stuff? Anything historically "weird" happening in Mexico, in the early 1960s?
maybe just for the lulz XD