In 1947, Yugoslavia received about 4600 Russian M91/30 PU sniper rifles as military aid, which were basically not used, as Yugoslavia had standardized on the 8mm Mauser cartridge. In 1952, however, a new sniper rifle was requested and the 91/30s were put to use. The Zastava factory took 4,618 M48A rifles of particularly good accuracy and fitted them with the rails and scope mounts from the Soviet 91/30s. Onto this, a domestic Yugoslav copy of the PU scope was mounted, and the result was the M52 (or M53, depending on the source) sniper rifle. The new optic was necessary because of the difference in ballistics between the 7.62x54R and the 8×57 cartridges.
It is unclear whether these rifles were all made and were actually put into service or not. In addition, some appear to have had the scope mounts welded into the rails and some had threaded muzzles for suppressors. Documentation is very sparse on these rifles, and I suspect there are some inaccuracies in the story as we currently understand it. Regardless, this is a very interested type of east/west hybrid design with a Mauser action and a PU optic!