This is the flag used by the Yugoslav Partisans in the Territory of the Military Commander in Serbia and in areas of the Independent State of Croatia where Serbs lived. It was adopted on 26 September 1941 at the Stolice meeting of the Yugoslav Partisans, and by a subsequent decree of the General Staff for Bosnia-Herzegovina. The source for this is Hoare (2006) [1], pp. 83-84. It was also used by the Yugoslav Partisans in Montenegro.
Own work
R-41