Theilig is the brand of Karlheinz Theilig of Leipzig, Germany. In 1955 Theilig released a derailleur called the Schaltfix, and followed it up, in 1956, with the Schaltfix-Super. Both were double pulley designs operated by a large lever and a rod linkage. I am entirely unclear about the differences between them.

During the 1950s, Leipzig was, of course, in the DDR - and East German cyclists of the time seem to have particularly fond memories of the Theilig derailleurs. This was perhaps because of their unique, East German, design but perhaps also because, unusually in the DDR, they were available as an after-market item that could be used to upgrade an existing bike.