This requires more context. Where did you find the document?
Dear Mr. Lawson,
Thank you for posting your request on History Hub!
The photograph is of the painting “Custer's Last Stand” by Edgar Samuel Paxson (1852–1919), painted in 1899. Without knowing the provenance of the document, it is impossible to say anything else for certain. Riley is too common a name for us to attempt to narrow down which Riley this might be referencing. We don’t have records of officer nicknames. Also, It is unlikely that “Chief” references a Native American leader. The term is used informally to refer to holders of various military ranks and positions, such as chief warrant officer or chief petty officer. “Sorry about that chief” may also be a reference to the catchphrase from the Get Smart TV series. “C.O.” is likely “Commanding Officer.” We don’t know what “EOP” might mean in this context. Experts in historical papers and ink might be able to guess at the time period if they examined the document in person, however the National Archives does not have the authority to perform this service for private collectors. We suggest that you contact a reputable antiques dealer in your area.
We hope this is helpful.