Date: Monday, October 2, 1871
Paper: Quincy Whig (Quincy, IL)---obtained on Genealogybank.com
However, this could be a good way to obtain some additional history on the 78th and perhaps get some information on the unit that I have not been able to locate elsewhere. Riley was only a private in his unit, so it is likely that he's not mentioned.
The paper did not state why those specific regiments were invited to the reunion, but I'm suspecting that those units (like the 78th) they were primarily comprised of men from southern Hancock and northern Adams Counties.
County historical societies may have items of this type organized in a type of vertical file, but manually locating these items would require page-by-page searches. That's something most genealogists don't have time to do.