This first style of Renak derailleur is the rebranded version of the FuS Optima derailleur in this collection. This design is also mildly interesting as it is different from that of any Fichtel & Sachs derailleurs that I know of, and is not obviously a copy of a Simplex or a Huret. It seems to be an original East German design.
The Renak (1st style) is an impressively finely constructed piece of equipment, with high quality finishes and tight tolerances. However it is a single pulley design - and so all the precision engineering in the world is not going to save it from mediocre (or worse) performance.
In this derailleur’s original form there was a splendid corrugated rubber boot protecting the sliding shaft mechanism. Unfortunately this had perished to oblivion by the time I got my hands on it.