Well, without the Pure Tone Trademark notice it doesn't appear to be a Max Keilwerth horn. Although Pete, just as an aside, it does have an octave lever shaped like that of the Hohner President.
Rule out Julius Keilwerth since it does not have the JK Best in the World logo, and what are we left with? Kohlert? I don't know, I'm not getting that vibe from it, but you know their really old horns better than I do. There are no FX or GH Hüller characteristics, so they're out.
I'm wondering if it isn't one of those obscure brands that was built through "heimarbeit", with parts from other suppliers. Just throwing that out there, since we know that was going on in Germany ATT.
Edit:
BTW, I just looked up my Max Keilwerth horns, and check out this
C.A. Wunderlich Alto 2101.
It is an early horn that he built, and it shares numerous features with this horn--including the shape of the octave lever, bell to body support brace, chromatic F# key, etc.
We know Max worked at home--heimarbeit--when he worked for FX Hüller, so perhaps this is one of early horns before he started the Pure Tone Trade Mark label. Just a theory...
I've also got other low # MK's in my gallery. They seem to line up in features as well. I'm thinking my first gut feeling was perhaps correct.