SDC is correct, the dash # indicates an engineering change. In respect to the 586, these are:
586 1981 introduction at AAA 000x serial prefix
586-1 1986 Radius stud package, floating hand
586-2 1987 Change hammer nose and associated parts
586-M 1987 Product warning by S&W: M overstamped to modification by factory or warranty station for models 586 and 586-1
586-3 1988 New yoke retention system
586-4 1994 Drill and tp, synthetic grips, change rear sight, change extractor
585-6 1998 Change in frame design to eliminate cylinder stop stud-eliminate serrated tangs, change to MIM hammer with floating firing pin. Change internal lock works
Discontinued in 1999 except for a special production run of the 586 L-Comp in 2000