I've found the source, which is a forum thread about motoring in Split, posted in 2011-2013. You were probably expecting "Leyland bus with body by Auto Karoserija Split" as the answer but I reckon this description is referring to the previous image in that thread, showing a bus in construction. The puzzle image is apparently only described as a bus operated by Promet, 1953 ("Prometov autobus na stajalištu u zagori 1953. godine"). I've posted both images here.