The gun does have the offset proof marks on the receiver. Did the aftermarket folks restamp the proof marks? I always thought that the offset proof indicated a factory gun, but in this case, the serial number places the gun too early for an original vent rib. I'm guessing a factory re-do.

You might try Pauline Muerrle (http://www.paulinemuerrle.com) to see if she might have records on the gun