Flat BP for sure, though stock could be either and Mauser used a lot of walnut stocks in 1939. In my trends many more walnut stocks than laminate have been recorded, but both are common. Volume one would have more information regarding period documents, and it may well be that in internal reports laminate was more widely used (more than walnut), but in actual observations walnut is more common so far as my research goes. Though we are talking dozens of rifles out of roughly 300,000 made in 1939.
Also, not that it means much, of the recorded q-blocks with matching stocks, both are walnut.