One of the three men appears to be Kaiser Wilhelm I, the other two possibly being Generalfeldmarschall Helmuth Karl Bernhard Graf von Moltke and the regiment's commander at the time, v. Grote.

This kind of print is usually referred to as a "Reservistenbild". Do you have any other papers relating to him? If not, due to the fact that he served in peacetime and that Prussian military records were largely destroyed in 1945, this is almost certainly the only document that confirms his military service.

The information for this regiment understandably is almost exclusively in German. Still, the following pages may be of assistance:

http://wiki-de.genealogy.net/Gren.R_9

http://de.wikipedia.org/wiki/Colbergsches_Grenadier-Regiment...

The 1889 regimental history has been digitized by google:

http://books.google.ca/books?id=6dEOAAAAYAAJ&redir_esc=y

This page consists of a register of names for the 1889 regimental history. Several are named Brandenburg. Could they be related?

http://www.kolberg-koerlin.de/militaer_index.htm