Magazine. From Max Popenker's site - note smooth magazine with cuts at the bottom and also shoulder rest attachment point on the stock:
"Normal" Lewis - note non-smooth magazine w/o cuts:
Guns were however most probably converted to .303 post-ww2, de facto 3rd caliber in Yugoslavia. Weapons in that caliber (machineguns mostly) were used largely by airfield defense units, and Brens (and Bulgarian ZB-33 converted to .303) were used until '70s due the air force having supply of .303.