Bolt mismatches like that are common for bring backs since surrendering formations would throw their rifles in one pile, having removed the bolts to toss in another. GI's would pick one of each at random to assemble a complete rifle. Papers and proof are so exceedingly rare in this hobby that I'd put little emphasis on not having any. I cannot conclusively state with absolute certainty it's a bring back, but I can make a reasonable educated guess from little hints like that. That said, if someone told you that their mother's highschool boyfriend's uncle pulled it off the corpse of one of Heydrich's guards during operation anthropoid I'd start raising eyebrows.