I'm quite interested in vintage Kawecos and recently found something I've never seen before. It is a small black model (not surprising) with a dark green ink window and a duo-tone 14 k gold nib. It is a piston filler with a blind cap and the piston mechanism is screwed in with a narrow ring the diameter of the barrel, just like old (pre-war) Dia models. I'm not sure but I think that the material might be celluloid. The barrel shows a clear golden imprint of the "Kaweco" logo and a faint imprint saying "Sonderklasse". Given these indications I would think it's a pre-war model but I've never heard of this line of Kawecos. Does anybody here know something about these models? I couldn't find anything anywhere.
Thanks in advance,