That is a nice straight up bolt mismatch, great find...I agree with the value assesments right around $700 + or -...might do a bit better on Gunbroker, but its a crapshoot there.
I wouldn't do a thing to the stock, wipe it down, call it good...if anything I wouldn't touch it with soap or water, moisten a rag with some blo and rub it in buff it real good...but it looks perfect the way it is...
Band spring is easy...if you have trouble finding one, PM me, I have some spares, and can get more...just make sure you get the 98k type...I think Yugo M48's or something similiar have ones that are too long, and they won't work....
Yes too on the Luft marked stock....they marked those like that until 1941 or so...H for Heer (Army), M for Kriegsmarine, L for Luft. Enhances calue a bit, but not tremendously....Kriegsmarine marked will bring a bit more than Luft...Heer of course are the most common.
If you tell what the WaA mark under the bolt root is, we can give you the maker of the bolt...