Too bad you forgot about that 24" damper John. I've never heard of one that large but it would be a beauty I bet. Griswold shows furnace size dampers of 8, 9, 10, and 12" for $3-6 a dozen in their 1915 catalog No.45. Catalog No.55 c.1930 shows them in sizes 14, 15, 16, and 18" for $22-55 per dozen. By the 1940-42 catalog the sizes remain the same as the No.55 catalog. Seems like furnaces were growing in size in these "later" years. Griswold also offered "damper clips" which had a tail piece also so one could connect the clip and tail piece to a round piece of sheet metal to make your own (custom sized?) damper. Catalogs give no indication of how large a damper could be made from these clips. 3" was the smallest Griswold damper I know about.
Ever wonder why most dampers have holes in them? 1940-42 catalog says "The present rule relating to manually operated dampers requires that they shall be such that they cannot close off more then 80% of the internal cross section area of the smoke pipe."
Steve