I have a build-up Model 12, in 12 ga., and the story that came with the gun was it was sent back to Winchester many, many years ago for a ventilated rib, but came back with the Simmons markings on the side of the rib.
Some research has revealed it is one of Simmons early ribs, by the words, and style of stamping from Simmons.
I can't find out too much more.