From time to time these pop up with TR insignia on so thought I'd post my new purchase up for info. They sometimes are sold as Czech or even Dutch side caps converted for use by the Wehrmacht.
This example is marked, as most examples are, B29 (Berlin Depot 1929) and E4 (for 4th squadron in a cavalry Regt). It would have had a cloth state cockade sown on the front.
The cap was issued on a trial basis in 1929/30 to a couple of divisions in the Reichswehr to try out. It was not adopted for general use but is the for runner of the 1934 side cap that did make the final cut.