About all you can tell about the frame is who made it...N= NEF and when S= 2002 in your case. The barrels are marked SB1 or SB2, not the frames. If you have a barrel for the frame, the last 3 digits of the serial number will be etched in the underlug, if it's a shotgun barrel, most likely it would be an SB1 frame. The only true way other than a matched factory fitted set, is to call or email H&R and give em the serial number, they'll tell you what barrels can or can't be fitted to it.
Tim