Quote from "Rifle and Carbine 98" by Dieter Storz:
"In February 1916, the Prussian War Ministry had to instruct state and private rifle factories to impregnate stocks with fish oil instead of linseed oil varnish. In the factory, stocks were oiled three times, requiring 1.90 kg of linseed oil varnish or 1.75 kg of fish oil for 100 stocks. Soon the replacement troops also had to make do with this disreputable product. But even this substance ran out after a while. In the fall of 1917, the Amberg rifle factory had to send out its rifles with unfinished stocks.[...] In February 1918, a decree of the Prussian Ministry of War ordered the introduction of Jäger's products “Kronengrund” and “Kronenöl” for impregnating gunstocks. However, their use was restricted to gun factories and gun repair shops.[...] The stocks were first rubbed with a cloth dipped in “Kronengrund”, leaving the recesses dry. After 15 to 20 minutes, “Kronenöl” was applied and the stocks were set aside to dry. “Kronengrund” was an oily nitrocellulose lacquer."
So in this case the "unfinished" stock would only apply to 1917 Amberg produced rifles and "Kronengrund/-öl" would've appeared after February 1918. A simple linseed oil varnish should do the trick in this case. The Mauser factory still had large walnut blanks, while other makers already had to switch to beech wood; having a larger stockpile of linsed oil in 1917 deosn't seem too far fetched IMO.