Hard to say. I think, based on the size of the ears, it's a Red Mountain series before they went to the automated method of making their CI. That would put it about 1930 to 1967 or so. I'm not sure when the "S" series of skillets were made, but they were smaller than their normal sized skillets. In other words, if that is a genuine 3S, it would be smaller than their 3.