[ithacaartist]

You might not have known of this "variation" because this configuration never existed IMHO. I think it's the result of putting parts together, along with perhaps a little creative marking. I have a vague recollection that Krieghoff serviced and re-sold guns like this, but unless it fits with that, its origin probably can't be determined.
I don't think you paid too much because its enigmatic nature makes it interesting, even if it turns out to be a Frankenstein/fantasy piece. If it shoots OK, then you should do that with it, and enjoy!