Thanks Dennis! I actually figured it out. CGB stands for College Green Barracks, which was a camp near Baltimore, Maryland, and seems to have been used as a staging area for Union soldiers paroled by the Confederacy, before they made it to Camp Parole, also in Baltimore. After I posted, I found I had a few other ancestors who had also briefly passed through College Green Barracks. Elsewhere in my family, they all seem to have gone straight to Camp Parole, but for some reason, this branch of the family (all from Ohio) did not.

I appreciate your help and willingness to share your knowledge. You obviously have a lot more experience than I do with these Civil War camps and Army procedures.

Mary