Hi Bill,
Your National skillet was a regular product produced by Wagner for many years in sizes 7-9 only. It was offered as a lower cost skillet to compete with other pans of lesser quality from other makers. The quality of National pans, however, seems to be every bit as good as regular Wagner skillets. Covers were made for the National skillets but are very hard to find. Griswold (Victor), Wapak (Oneta), and Favorite (Miami) all made similiar skillets which are slightly smaller in diameter than their regular skillets and a bit lower, too. National skillets are probably priced close to Wagner's regular skillets and are farily plentiful. Only Griswold made these pans in sizes other than 7-9. They made the rare No.5 and 6 sizes. No covers, to my knowledge, were made for any of those other makes of pans.
Steve