Musee de l’Armee des Invalides, Paris, Dec. 2017, behind glass. Not sure why a straight bolt.
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I don't think a good answer can be formulated with what we have...the pics are crap, grainy, fuzzy, can't make out details, and key pics are missing...
As Flynaked points out, if the bolt matches, and has round gas holes on a byf 42...something odd there...could be a humped bolt. Bruce points out the stock is not MO, another strike, and the inletting work around the takedown lever looks very crappy, not something you'd expect from a German armorer or factory, not withstanding the one off hand made theory...
Now, on the plus side, to me, it would seem that machining that interrupted thread attachment and having it fit tightly and properly has got to be very, very difficult without lots of the right equipment and know how...although I am not a machinist, maybe Fly can speak to that....
I guess there is the slim possibility it is a real take down, that had the stock either lost or so badly damaged someone tried to whittle a replacement kind of poorly...
I would say without much better and more detailed pics, at this point you'd have to go with a fake or put together...but it will be interesting to see how this plays out...
I think you are right. I took a close look at the stock and it appears to be sanded and the numbers restamped. The stock is flatten and below the buttplate. Also the stock is broken out under the takedown plate. The fit of the trigger guard is not up to factory. All the evidence leads to a replacement stock. The original stock was damage/broken at the takedown breaking all the takedown ears and breaking the lever which was welded and a new stock fitted by not a too good a stock maker. This answere a lot of questio.
Interrupted threads are not that hard to do. You are just basically removing metal. Not saying it would be easy but not really requiring a lot of skill.
You think the threads were the original barrel and receiver threads, and they just had to be milled out correctly to work? If thats the case, I can see it being doable I suppose...
It doesn't really answer any questions, its just a possibility...a maybe...someone with real knowledge would either have to evaluate it in person, or lots and much better, clearer pictures of literally everything would need to be posted for evaluation...
Way too much at stake value wise, one way or the other, just to say good theory, case closed....