All the information taken from original rolls stored at the National Archives has been placed in individual file service files (CSRs). It's indexed by name and unit. That makes it easy for people to find and use. If you really want to see the original U.S. rolls, you've probably need to plan a visit to the National Archives. For most people that's a significant investment of time and money.

Why not start with the soldier's name and match it to a U.S. Volunteer service file? If you know the soldier to be a Confederate prisoner sent to Rock Island, that will narrow your search. We can copy his U.S. service file which should include all the relevant discharge data.