Regarding the Village of Sommin and Province of Pomerania, both are correct.
What is not correct is the "country of West Prussia." It should be the
"country of Prussia," without the "West" part.
What has probably led to confusion in this matter is that the village of
Sommin, south of Bütow and Stüdnitz, was located at the border of the
province of Pomerania, where it meets the province of West Prussia. The
family was "officially" living in the province of Pomerania but easily could
have gone to church or to market in the province of West Prussia. Both
provinces were probably important to their lives when they lived in Prussia.
Notice the document you have says "all Prussen service" indicating the whole
kingdom of Prussia. It does not say "all Westprussen service."
If you are in the U.S., it was like living on a county line in a state.
Pomerania and West Prussia shared a common border (like two side-by-side
counties) but both were in one state (Prussia.) At the time your ancestor
lived there, Prussia was a very large kingdom in which were many provinces --
including one named Pomerania and one named West Prussia.
The picture will be easier to understand if you can see an overall map of
this area. One excellent new source for this is "LANDS OF THE GERMAN EMPIRE
AND BEFORE" by Wendy K. Uncapher and published by Origins, Janesville, WI.
There are many other equally good map books that cover the German Empire of
1871 and show the individual provinces. They do not show individual towns,
however, and one needs a good-scale map (such as 1:300 000) to identify and
locate small villages.
Regards,
Karen B. Whitmer