Sue, it's probably milk paint. The only current manufacturer of this product, The Old Fashioned Milk Paint Company, has a great site telling about its history and use. I believe that the site is www.oldfashionedmilkpaint.com. The chemistry of milk paint is totally different from that of common paint today and that's why most strippers will not remove it.
In any case, the best remover is Peel Away 1. This is available at Sherwin-Williams stores and other sources. Their site is www.peelaway.com. Be sure to follow all the directions and safety instructions exactly.