According to the older book I'm looking at, the serial number dates it to 1984. Any markings on the back strap (RHKP)? Could be a Hong Kong Police Gun. They came into the US several years back as surplus. It could have left the factory with the swivel attached. Only an Archive letter from S&W will tell the story.
Smith & Wesson Historical Foundation - Letter Process - Insuring that the rich history of Smith & Wesson will continue for generations to come
This is a Square butt K frame, so any grips so marked will fit.