This is an example of what is referred to in the books as a "copper colored" frame.  There is disagreement as to what caused this.  Some say it was an experimental finish.  Others say it was a bluing solution problem.  I subscribe to the latter.  This finish shows up on at least two runs of guns.   The finish held up so poorly that every example I have seen has virtually no grip strap finish left.  I discussed this with Warren Buxton once and he said that he had seen examples where you could virtually rub the finish off with your finger. If the finish didn't hold up well it doesn't seem to me that Walther would have tried it at several different times.  This is the earliest run of Walthers that I have seen this finish on.