1945 Split local provisional issue, No. 7 on pelure paper, 20 kn / 3.50 kn, block of four (positions 9-10 / 19-20) with exceptionally rare and attractive error of overprint on partly printed base stamp.
There is no print on the backside, so this is not a classic paper foldover error. Therefore, I assume that a piece of trash paper accidentally came in between the sheet and the printing plate. Such items are rare because they should passed two inspections: first after the base print and second, four years later, after the overprint.
A possible situation...
