The no-dash and -1 Models 586, 686, 581 and 681 had a problem with the firing pin and bushing design that allowed primer cup material in the case of certain high-pressure loads to flow back into the gap between the two, potentially binding the cylinder. S&W redesigned the parts (the redesign being incorporated into the -2 variations) and issued a recall for the earlier guns. Guns that have been sent back to have the new parts installed are stamped with an "M" in the frame cutout near the model number. S&W will still make this modification at no cost and will cover shipping both ways. The appearance of the gun is not changed in any way.
NRA Benefactor Life Member
NRA Certified Pistol Instructor
Pemigewasset Valley Fish & Game Club