The top marking is the cypher, the Prussian King, who was also the Kaiser. The Crowned/D looks suspicious to me, odd anyway, they did use Latin letters late in the war, so it could be ok, but the other acceptance stamps would be a place to start for an analysis (original maker-date of the rifle the stock came from).
As for the SCW, a fascinating subject that could occupy your life for years, - it is possibly the most distorted conflict of the first half of the twentieth century, as much due to socialistic "liberals" like Hemingway and Orwell propagandizing the "Republican/Loyalist" cause (far left or full communistic is a better label for them... legitimate they never were, their political accession to power were full of political gimmicks and violence/assassinations to achieve power, hardly a free election...) as the demonization of Franco and the Nationalist alliance.
As for small arms used during that conflict, little is known with certainty regarding the intricate details, - all of the activity was illegal and the small arms came from so many places it boggles the mind. Even Yugoslavia and Greece played a significant role, ironically Greece with its rather friendly relationship to Germany supplied "German" rifles to the Republican forces, - apparently the Greeks never really change, as is often pointed out throughout Greece's history Greeks would fight (mercenaries) for anyone, even fighting other Greeks, Spanish gold was enough for them to smuggle German rifles to a government they could have had nothing but contempt for...