Stock is defintely a Soviet made stock. It is not Finnish. Likely a Continuation War 1941-1944 capture 91/30 donated its stock to this action. In fact, it is very likely a 1941 production stock the Finns have retro'd onto this particular action. Reason? It still has the early screwed escutcheon but has the notches behind the lower band to ease removal. This feature begins to show up on late 1941 production Soviet made rifles and carbines.
As far as the thinned buttstock is concerned, I'd be willing to wager it was thinned by the Finns at the time of issue so the Finn horseshoe swivels could be attached--a lot of Soviet rifle stocks are both too thick and too deep for these swivels to fit properly and so the natural solution would be to thin the lower portion of the stock so they'll slip into place.
My two cent's worth....
