I sure hope the OP resolved his issue 2-1/2 years ago.
Forget the rollmarking. If your serial nuber ends in 'M' it is a Magnum receiver, for 3" shells max. If the serial number ends in 'A' it is a SuperMag for 3-1/2" shells. Anything is possible with rollmarking, but I have not seen a SuperMag receiver that did not say SuperMag on it.