The Bann belonged to the West Ruhr-Niederrhein region until August 1938 and then to the Mitte Mittelland region. Since they are brown boards that were manufactured until 1938, it can be assumed that they belong to the West Ruhr-Niederrhein region.
If you're thinking of adding a triangle to the straps then a West/Ruhr-Niederrhein would be the sensible choice. I would just note that Bann 234 was also within Gebiet 34 Düsseldorf for a short period after the division of Gebiet Ruhr-Niederrhein in August 1938 so theoretically it's possible that this triangle was worn by some during that period.
Another thing that would give you a further triangle option would be supply problems which resulted in many Gebiete being unable to provision the new black shoulder straps until 1939. Düsseldorf and Nordsee would be just two examples of this with the former issuing an order in Dec 1938 stating that the brown straps were to remain in use until Spring 1939 at least. For Bann 234 which had of course been within Gebiet Mittelland since Oct 1938 this means that you could pair your brown straps with a Mitte/Mittelland triangle and still be able to justify it