Included in the application of Sandburg is a portrait of Sandburg, shown in this post. Also included is information on Sandburg's travels outside the United States (only to Puerto Rico), why he is travelling to Europe, and information on his place of birth and his father's citizenship status and nativity. Letters are included from Sandburg's employer at the time and his mother. Parts of those letters are included in this blog post. Sandburg even includes the place of birth for his father.
Sandburg, like others, signed an oath of Allegiance.
Part of the letter from Sandburg's employer is shown here. The rest of the letter summarizes Sandburg's employment history and discusses his time in Puerto Rico.
Sandburg's mother even signed a letter in his behalf. In the first part of the letter she discusses Sandburg's date and place of birth and information on his father.
Sandburg's application for a passport can be viewed on the Ancestry.com site.
These US Passport Applications and images 1795-1925 have been indexed by the name of applicant by Ancestry.com and are searchable. Those without Ancestry.com access can obtain a free trial to Ancestry.com to experiment with this database and others.