If it is mostly matching and not horribly worn, the chances of it being a legit German rework increase greatly.
My previous comment was predicated on it being an import, Eastern Europe/Balkan/Albanian, used post war, often largely mismatched when imported. Those folk would have counterbored it, while 3R Germany would've (likely) rebuilt it with a new barrel, IME.
Imperial era was far more frugal; I've seen bolts serialed 3 times, barrels with 2 serials and 2 clocking marks, mismatched parts that are clearly original to the armorer rebuild assembly, X'd out/reserialed bits, et.al. You really don't see that much use of salvaged parts during the 3R era, outside of SS reworks of Imperial/Weimar rifles, and the later war use of armorer parts, likely evacuated from eastern region depots.
Does anyone have a legit example of a 3R era rifle being counterbored? If so, is there inspection markings to reflect that?
OP, please load some more images. If you can get measures of depth and diameter of said counterbore, all the better.