Simmons and PolyChoke, possibly others, installed aftermarket ribs on Model 90's, so it might be possible that they removed the matting before attaching the rib. Some Simmons ribs were marked with the company name. Maybe PolyChoke also marked their ribs.
i have a very early 12 ga Sears Ranger 103.1 (Model 90) that does not have matting on the topbarrel. It also has the first generation forearm, and the top lever is attached to the frame with a visible flat head screw.