I believe you are correct. This particular rifle was brought back to the States by my brother-in-law when he returned from Viet Nam in the late 60's. Surely it was a Russian capture during WWII and then given to the NVA. It has all the typical earmarks of such a rifle.... missing front sight hood, missing cleaning rod, missing capture screws in the trigger housing and, as you said, mixed parts. Thanks for confirming the "turned down bolt" issue!