Stevens 5100

I have been bird hunting with a Stevens 5100 16 gauge and love this shotgun. After doing some research in the forum and on other sites, I am still confused as to how old this gun actually is. The numbers on the barrels and the receiver match and begin with two capital A's followed by a four digit number. The receiver does have the engraved setter on the side. I certainly am not looking to sell this gun I was just really curious as to the age of it. Any help would be greatly appreciated.