These cards are Deutsches Reich MiNr. P308 overprinted for use in German-occupied Ostland.
This set was issued on 4 November 1941 for use in German-occupied Ostland. The stamps are Third Reich MiNr. 781-789 overprinted “OSTLAND”.
Shown below are the stamps of the GermanStamps.net Collection on the physical album pages.
Upon the invasion of Russia, Germany established the Reichskommissariat Ostland to govern Estonia, Lithuania, Latvia, eastern Poland, and western Belarus.