<but I don't know whether they did any switching when a rifle was arsenaled.>
I bought one from AIM that had a bolt/receiver from the walnut era, a beech stock dated after the walnut era (of course) and a magazine dated even later than the stock. I assume this was a re-arsenalled gun.
However, everything fits together well and functions perfectly, so no complaints here. Doesn't look bad at all, once I cleaned the stock a bit.
There were fewer beech guns than walnut, FWIW.
Esthetically, an all matching walnut piece would be my first choice but I buy guns primarily to shoot them.
You can't miss (pun intended) with any K31.
Bill