The linked vent rib Model 121 has the very early Simmons rib matting pattern. I am guessing late forties to mid fifties. By the way, the Simmons rib installation on the Model 61 Winchester was not a catalogued Winchester item in my experience. It is an item offered in the Simmons catalogs of the mid to late fifties and extremely few were built. The 121s would be a neat buy at a bit less than half the asking price.