According to the SCSW (Standard Catalog of Smith and Wesson), Smith and Wesson acquired Alcan (Alton, Illinois) in 1972. They manufactured all of the standard gauges of shotgun shells, as well as pistol and rifle bullets. Some of the boxes are stamped "Fiocchi."
Most of the boxes are blue with white lettering, or white with blue lettering. Many of the boxes indicate "Member Smith & Wesson Sporting Goods Group." A slightly different (and perhaps unique) box in orange and black with white lettering states "S&W Ammunition Company" with the name "Power Mate." All of the shells had an S&W headstamp.
More specifically, in response to your question of when they manufactured shotgun shells, it probably lasted from the early 1970's into the 1980's (but I am sure one of our forum members with expertise in this area can be more precise).