flag of auxiliary ships of VM of the German Democratic Republic
https://web.archive.org/web/20191109223946if_/https://i.ebayimg.com/images/g/HuMAAOSwVTldnK0g/s-l1600.jpg

Jugendlexikon. a-z. 10. Auflage. VEB Bibliographisches Institut Leipzig, Leipzig 1981
User Jwnabd from German-Wikipedia