Quick over view of what you ask about.

The Yugoslav M48 Mauser rifle was manufactured from 1950-65 at the Zavodi Crvena Zastava (Red Banner Works. Zastava was known as "Preduzece 44"--meaning Enterprise 44, from 1945-52. In 1952 it was renamed Red
Banner Works.) The M48 series came in four versions--the M48 (all milled parts) M48A (stamped magazine floorplate) M48B (increased use of stamped parts) and the comparatively rare M48BO
(the BO=bez oznake, meaning "without markings.") The M48BO versions were new manufactured M48s as well as numerous captured, reconditioned and "scrubbed" German Kar 98k rifles (sometimes the latter were called M98/48N, the "N" standing for "Njemac," which is Serbo-Croat for "German.") The M48B/M48BO versions were made chiefly for export--and most of these were sent to Egypt.

If it is a German made rifle.  It will have no wood between the receiver ring and the rear sight.  If it is Czech, FN, or Yugo made, there will be wood there.

The BO can be German, but these are very rare.  This i believe is the M98/48.  Not many of thosse around.  Most likely it will be an M48 varient Usually and M-48-A, that has had all markings ground off.  Most, but not all, Yugo rifles have  a hard wood stock.  No Yugo made rifles have the bolt take down hole in the stock, but not all German rifles do either. Late war issue rifles do not have this. there was also one year before the war as well, and these had walnut stocks.

All Yugo military rifles will either have a crest, Republic or Commie, or manufacturers mark on left side of receiver ring.

Just picked up two M-48's this past weekend for $210. 95% condition but the bolts were non-matching.  GREAT project guns.

The 99% 1903 M-96 Swede was a nice buy too.