So I've been reading my copy of "Rifle and Carbine 98" by Dr. Dieter Storz this weekend. FANTASTIC reference and the only comprehensive book on the WW1 German Mauser. I highly recommend it (despite the ridiculous cost - I gave $145 for my copy plus shipping and duty, came to about $200) ISBN:3-902526-05-X.
I've now changed my mind about what the stock finish is - I no longer think it's polymerized linseed oil. Evidently Danzig (and the other factories) ran out of linseed oil in 1916 and 1917 depending on the factory.
For a few months they used "train oil", whatever that is, and then in February 1918 they settled on a 2-part system called the "Jager Kronengrund/Kronenol" process.
First the stocks were rubbed with rags dipped in Kronengrund, an oil-bearing nitro-cellulose varnish, then 15 minutes later they were rubbed with Kronenol which contained a polymerized oil and grease. This finish cured to something resembling french-polish shellac or a well-rubbed linseed oil finish that has polymerized over time.
The giveaway for this 2-part finish is that the inletting is dry and untreated by any oil in this process, which matches my carbine perfectly. This was the predominant rifle finish from February 1918 to war's end, and my rifle was made in 1918 so it all fits.
Worthy of note is that "train oil" was an ersatz finish and some stocks went to the front totally untreated when train oil ran out in late 1917 (remember that naval blockade thing?), but the Jager process was not an ersatz finish - it was approved to officially replace linseed oil by the Kaiser and the intent was to continue using it post-war, though the cessation of all arms making in 1919 evidently put an end to that idea.
Nifty, huh?