Thank You for all your replies. I've wondered about this for a very long time. I have 2 slimline frames in my attic, one of which is horribly butchered and will likely be scrapped. I bought an Atlas for $250 back around 1977 that was running but a cosmetic disaster. When I lifted off the tank I saw that somebody had chopped the top rails off and welded in a single tube, and then somebody sawed that off and re-welded some top rails back in. Neither welder knew what he was doing. I probably should have thrown it away decades ago. Maybe someone is interested in the back half of a slimline?