According to genealogical research, my great-grandfather emigrated on the SS Coriolan in 1869 whose port of exit was Bremen. Records indicate that my great-grandfather was from the city of Margorin or Margorum, but I am unable to read the handwriting and unable to locate any city with a similar spelling in Prussia. Did such a city exist in Prussia at that time?
Passagierlisten.de is a register of passenger list from the port in Bremen. While it does not have passenger lists from before 1900 (they were lost in WW2) it does feature a list of towns of origin, which amongst all others also includes Margonin.
One you know this exists one can simply look in up the Wikipedia entry for Margonin.
Margonin [marˈɡɔɲin] is a town in Chodzież County, Greater Poland Voivodeship, Poland, with 2,941 inhabitants (2004). [...]
The area was included in the First Partition of Poland, at which time the area fell to Prussia, and was then part of the Grand Duchy of Posen until 1848, except for the Napoleonic period of the Duchy of Warsaw (1807 to 1815), and then part of the Prussian province of Posen until 1918
Since Posen used to be in the east of Prussia that's likely to be the one, and it does still exist as a small polish city of some 3000 people.
However, East Prussia in 1945 is pretty different from Prussia in 1869, and Morag is pretty far from Bremen. In 1869, I believe all the territory colored blue, green, and light blue, could be technically considered "Prussia".