Flag of the Province of Pomerania, used in 1882–1935
Own work, based on:

Prussian Cabinet's order of 22 October 1882 (Deutscher Reichsanzeiger 1882: no. 264)
Meyers Konversationslexikon, 1889, 13. Band, S. 65
proportions: FOTW Flags Of The World website
  lightblue colour CMYK 100/30/0/0 – original research
vectorization: JDavid