I see the adjustment as being to perhaps tension a spring, certainly to be able to retain one that was there. Also, the cuts on the adjustable end could be part of spring retention and the shape of the adjust end could indicate a guide. I have a number of Mosins. While they have long firing pins if it helps I can confirm its not from one of them. Since its longer and more robust I'm wondering if its from some kind of a heavy machine gun or other high caliber device .. will need smarter people than me though to confirm / disprove that thought.
good luck on the search.