You can tell this rifle was taken care of pretty well as indicative of the slots in the screws. Most times they are messed up to some degree.
I believe, as Whig alluded to, that your stock can be brought back some with boiled linseed oil after some gentle cleaning. I can't tell you exactly the right way to do it, but I'm sure somebody here can. I worked on a 1916 Spanish Mauser not long ago (rifle cost $145) and it was an improvement. Of course I wasn't worried about impacting the value of this Mauser.