Thanks Garry and Wilhelm for their info. My blue Truppenarzt (1935 - 1938) boards and my blue Oberfeldscher (1938 - 1945) boards are 100% original and do not have a brown back. I have no doubts about the two pairs.
Rules were certainly broken easily if there were no major changes. In many pictures you can see strange pieces again and again. I've seen some pictures where it's clear that the Hitler Youth leader was wearing an embroidered RJF triangle. And it always looked different.
If one were likely to have such an original triangle, many collectors would likely mistake it for a fake as it does not conform to the woven norm.
Of course, the fakes are getting better and better, but you also have to follow many facts and trust your feelings. Style, fabric, metal thread and seams are very revealing.