I never took the time to measure one, but you probably could use the rear band spring from a "wide" RB equipped G98 stock. They look about the same, but I have never tried to substitute them. You probably could alter or adjust (grind) one to fit, I think this was done, or at least some seem to have been ground to fit (I assume period, but recent also, I went through my database looking for RB spring details, no good examinations of springs, so i can't help with measurements or good pictures)
Pictures attached of DD 1924 for a comparison to the real deal,