Twelve years ago I purchased a second hand Stevens model 79 pump 12 gauge shotgun with a 30" barrel and full choke. The serial # is 0982XXX. It has a couple proof marks on barrel near the receiver. One is stamped 81 inside a circle. The other is S8P stamped inside a circle. Both marks are on top of the barrel by the rib. Does anyone know the approximate manufacture date of this shotgun? Any help would be much appreciated. Thanks in advanced!