Nowadays in Central Europe "Karpatenbüchse" seems to be another, romantic or nostalgic - and commercial - name for a Kipplauf SS rifle, usually a short Stutzen, such as Merkel's K4: https://www.jagdzeit.de/artikel/deutschland-k4-merkels-karpatenbuchse-ausgabe-39

But in my mind, and in prewar catalogs such as Gebrüder Merkel if I remember correctly the meaning is a light moutain rifle with a rather longish barrel and 3/4 stock (inbetween halfstock and fullstock).

With kind regards
Jani