Founded in 1864, the J. Walter Thompson Company is the oldest and still one of the largest advertising agencies in existence. These are examples of self-promotion done by the company: 2 "house ads" to publicize their talents to potential clients and another ad making statistical claims.
Images in this section from the:
Advertising Ephemera Collection
Emergence of Advertising On-Line Project
John W. Hartman Center for Sales, Advertising & Marketing History
Duke University Rare Book, Manuscript, and Special Collections Library
http://scriptorium.lib.duke.edu/eaa/