Bought that stock some time ago as I needed a replacement. The black discolorations are quite often seen in Germany. They are caused by the guns being hidden away in most unlikely places for several years after 1945. As steel parts rusted, iron oxide impregnated the walnut stock wood and stained it black in places.
http://forums.nitroexpress.com/showflat.php?Cat=0&Number=329233&an=0&page=3#Post329233
By accident I got another Mauser stock with the M-S stock and put that to some use too.
http://forums.nitroexpress.com/showflat.php?Cat=0&Number=329611&an=0&page=2#Post329611