You can use footnotes or endnotes in Chicago, both of which are numbered consecutively. Footnotes appear at the bottom of each page while endnotes appear at the end of your paper. In a Chicago-style note, the title of an individual item should appear first in your citation. If an items does not have a clear title, you may have to make one up, which is permitted for archival materials. Other pieces of information that could be included in the note are the item's creation date; the box and folder you found it in; the name of the collection it is a part of; and the name of the library, archives, or other institution that owns the collection.
1. Photograph, "Japanese Sentry, Hankow," [ca. 1945], Box 1, Folder 1, William H. Sager China Marines Photograph Collection, Special Collections and Archives, University Library, California State University, Northridge.