Well... The fact that Max made horns for FXH is nothing new. It's an established fact--as is his production of horns for Oscar Adler. I wrote about it years ago. That said, until I happened across Bluespeter1's Max Keilwerth page about a year ago, I had never seen anything making a connection between MK and the Champion model of FXH horns. I would be curious where he got his info.
I've seen the Champion model and some variants for years. They all had that rather distinctive bell-to-body brace. So, minus the Max Keilwerth part of the equation, you're thinking that the Champion is FX Huller?
Oh ya, there's no question about it. The Champion was F.X. Hüller's base model, the World was their intermediate model, and the World Super was their pro model, and the World De Luxe was their top-of-the-line model.
I can try and see if there is an old company brochure outlining the models that I can take a photo of. I don't have PDF I can upload a page from.
I checked and there is no brochure, but there is a 2 page spread--in German of course--of a 1945 brochure outlining the various World models (World, World Super, & World De Luxe) and their improvements. I could photograph that.