From experience looking at these guns for many years, as well as referencing George Madis’ book, The Winchester Model Twelve, and Dave Riffle’s book, The Greatest Hammerless Repeating Shotgun Ever Built: The Model 12 - 1912-1964, it is clear that Simmons markings were never placed on a factory Model 12. Simmons designed and sold to Winchester the rights to produce Simmons’ designed ventilated ribs.
The plot thickens a bit when one considers the Model 42 ventilated ribs. Unlike Model 12 production, the third variation Model 42 ribs were built and installed on Winchester barrels by Simmons, and were marked “Simmons Patent” on the rib. Once these ribs were installed, they were shipped back to Winchester for final fitting and finishing.
The upshot is that a Model 12 with Simmons markings is not a factory gun. A Model 42 with Simmons markings may be factory.
