Amateur sleuthing is fun!
A cursory search of tandem axle box trucks suggests that the box in the picture looks much more like the box on a M109/M135 truck. The two separated cutouts for the tandem axle wheels are quite characteristic. Most eastern bloc vehicles, such as Praga, Tatra, Ural, ZIL have either one cutout spanning both axles or none at all. CCKW is out for the same reason. The box itself has a mild radius from the side walls and rear wall to the roof. This is also similar to the US trucks. Eastern bloc trucks seem to prefer a cut edge angled design, a right angle edge or a "hoop barn" style design. The box may well be something more uncommon; it is not a water purification box (does not have the front overhang). Maybe a shop box of some sort.
The truck itself has a hood, so cabovers are out. No Unimog, Pinzgauer, Robur, IFA or similar.
I am leaning towards a weird M135/M44 series truck.
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