If the Mdl 12 has a factory rib there will be a proof mark just to the left (as I recall) of the rib close to where the barrel attaches to the receiver. In the office now but I can check later if you want to know for sure. The plain barrel mdl 12s had the proof mark on top of the barrel, that proof mark will be covered up if a rib was installed other than at the Winchester factory. There were some factory authorized Simmons ribs as I understand it. I have seen Simmons ribs on Mdl 12s that had the offset proof mark. Of course there could have been a factory rib removed and then replaced with a Simmons.
