1966.
Does it have a flat cylinder release latch? It was phased out during this year. M12s with flat latches command a slight premium, and are especially unique because IIRC the M12 is the only civilian K frame variant that normally had one.
{EDIT} Footnote: Many years of conventional wisdom says that you shouldn't ever fire +P ammo in a K frame .38Spl Airweight such as an M12, but a popular ongoing thread on the S&W forum has proven that the gun can withstand over 10,000 rounds of it. I'll let you and your MIL draw your own conclusions.
