Ok I am vastly over simplifying things, plus I am missing a good bit of history in between like King Peter II Vs Tito and so on.
The Germans invaded
Yugoslavia in 1941 and stayed there until the end of the war in may of 1945. So it would not be far fetched to see your 1944 made Mauser in the hands of a
German soldier sent to Yugoslavia as part of the occupation force in 1945. Either by surrender or battlefield pick up, your Mauser found it self property of the Yugoslavs in 1945. Some time after 1952 your Mauser underwent over haul and was put into WWIII storage. However it is interesting that your barrel has been replaced by a locally produced one. If it could talk it would probably have an interesting story.
Moving on. When the Germans surrendered they left all of their equipment there from their rifles to their canteens and zeltbahns. The Yugoslavians basically stripped them to their shirts and pants and then let them go home unlike the Russians.
Yugoslavia and the USSR did not have a good relationship after ww2. The government of Yugoslavia went what is loosely called “ The third way.” Which was a softer version of communism with connections, trade and limited travel to the west. This along with the fact the Yugoslavians would not allow the USSR to station troops or build military bases in Yugoslavia soured the relations with the USSR. Tito did not want to hand over power of Yugoslavia to moscow. In retaliation Stalin did not supply them with any cash, arms or support.This is why you don't see Yugoslav used mosins and their air force used American f-84 thunderjets until they got migs in the 1960s after Stalin was long gone.
So after ww2 with chilly ties with the west and no ties to the USSR and the rest of the eastern block, they recycled everything the Germans left there down to the last zeltbahn button. The German weapons were convenient because they were already using the 8mmJS as their standard cartridge. Along with the weapons they fitted their army with German helmets, zeltbahns, boots and other gear well into the late 1950's and some times even longer. If it is not broken, use it until it is seemed to be their motto.
Odd fact of the day: The movie Kelly's Heros staring Clint Eastwood was filmed in Yugoslavia. This was because it was one of the few places that had enough German equipment to outfit hundreds of extras as German soldiers.