I would guess.. it was experimentation. if memory serves, Yugoslavia didn't start chrome lining barrels until the M70 AKM. So it could have been method testing the best way to apply the chrome lining. The long barrels were prototypes anyway, so why not experiment on a prototype, screw it up, oh well. I'm sure it's eaten a fair share of rounds as well.
Geeze,, now I have to check mine at some point, I never really bothered to look.
One variant I've never seen in person was them Bosinian imported ones, they have a "new" chrome lined barrel as I remember.