WWII Occupation - Serbia Revalued Airmail Overprints On 28 July 1941, Serbia issued these revalued overprints of Yugoslavian airmail issues. Set Date(s) 28 July 1941 Watermark(s) None Album Page(s) Revalued Airmail Overprints On 28 July 1941, Serbia issued these revalued overprints of Yugoslavian airmail issues. Set Date(s) 28 July 1941 Watermark(s) None Album Page(s)