Ah, the new numbering system tells you NOTHING other than it is a Remington Shotgun - RS. All Supermagnum 3-1/2" receivers are supposed to say that on the receiver, but I wouldn't trust that. Get a FIRED 3-1/2" hull with an unfolded crimp, and put it in the chamber and see if it will eject. If it will you have a Supermagnum receiver. The Supermagnum receiver is different than the 2-3/4" and 3" receivers, but the barrels will interchange.
My bet is you have a regular 3" Magnum receiver, but NOT a 3-1/2" Supermagnum receiver.