The case is solved. I stumbled on a Swedish bicycle site and found this. I contacted the site owner, and he told me that it is indeed a Crescent Helracer 200, most likely from late 1930's. A fine example of a similar bike can be seen here. It'll take some time before mine is as beatiful...

It should either have no rear derailleur at all, or a Fichtel & Sachs derailleur. As those are virtually non-existent these days, I will build mine without any kind of rear mech.