Thank you, Corinne for additional information which was very helpful. Margaret - what a wonderful painting - you are so lucky to have that treasure!
So, I finally found some family information that might be relevant, but I have no idea if it pertains to the subject in the photograph.
In 1913, my GG Uncle (Abraham Sladowski) living in East Prussia was seeking Citizenship. A reference by a local official in East Prussia wrote a letter of recommendation for him. In this letter it states "The eldest son is naturalized, served in the 52nd field artillery regiment and was released early".
I see that the 52nd field artillery regiment has the following details in Wikipedia (below). Would this information fit in any way to the person in the photo who was born in 1861 in Poland and lived in East Prussia, near Tilsit, from about 1890 onwards?
Thank you all for your patience with my questions.
Ann Arbor, MI