Penal battalions are military formations consisting of convicts mobilized for military service. Service in such units was considered a form of punishment or discipline in lieu of imprisonment or capital punishment. I have not seen a penal military unit cancellation before...
1945 Split provisional issue, stationery of type St.2IIa, additionally franked with 1 din, sent from Skradin (21.VII.45) to Položaj (military position), censored by the military censor. Handwritten Natrag (Return) and Kažnjenik (Prisioner). Rare penal battalion cancellation (ŠTAB DISCIPLINSKOG BATALJONA / IV. JUG. ARMIJE). Late usage. This is an extraordinary item.
