It seems they did both styles on both sizes. That Chestnut Coppertone I posted from the age page is a 22 and it has rounded corners. One must be early and the other a later one. The sky blue is rare in that it has both the utility pan and the ash catcher disc. Could have been sold as a '57 model with a '58-'59 disc. Or maybe the original owner bought a disc for it when they came out. Who knows..