In 1987, seven years after the release of the Model 586, there were reports of cylinder binding with some types of standard .357 Magnum ammunition for L-frame revolvers manufactured before August of 1987. S&W put out a product warning and authorized a no-charge upgrade to make modifications to all 586, 586-1, 686, 686-1, and 686CS-1 revolvers.
The way you can tell is by swinging the crane out to reveal the serial # and model #. If it has been modified, it will have an "M" stamped between the two #'s. Really only applied to no-dash and series 586-1.