Bulgarian Prime Movers during WWII:Hanomag SS-100 with Flakvierling 38

A single Flakvierling 38 was captured in early April 1945 by forces of Bulgarian 16.ID during fighting on Mur river.It was transferred to the 20 mm. AA battery of said division and participated in bringing down an enemy Bf.109 in the area of Chakovetz later same month.
There is a picture showing a battered Flakvierling towed by Bulgarian Hanomag SS-100 tractor on the street of a big town.
I was inclined to think the town is Sofia, but a friend of mine suggested it might be Belgrade(the soldiers standing by are definetely not wearing Bulgarian WWII uniform), so the picture might show Bulgarian forces on the way home after the war ended.

A story and research(which is not over yet) worth a build!