If you were to research lamps, manufacturers and importers in the US in the latter half of the 20th century you would realize that there were hundreds of companies with hundreds of products in many styles and colors. Manufacturers would buy porcelain from all over the world and build lamps and fixtures. Rembrandt lamp would be a good one to start with because they specialized in large traditional forms like this one. Stiffel was, and still is most likely, primarily brass finished base metal with porcelain from Lenox occasionally that was usually off white. I wish you luck in your search but it will be arduous.