'SA' mark does not necessarily indicate capture..that marking didn't come along until 1942 and is an ownership mark.
However, looking at the pic again, I may be incorrect about the German mark.
Yes, you are correct. It was either captured by the Finns during WW2 or purchased from Germany during WW2. Since it is a 1927 model we can rule out anything from WW1. But who knows the original stock was most likely damaged and had to be replaced. The Finns kept any and all spare parts that were usable. Some of the stock may have came from a old M91 from WW1 and they finger spliced it to a new forearm. The Finns were like they Indians, they used the whole buffalo