Post Office Consolidation Act of 1872
The Post Office Consolidation Act of 1872, formally entitled as the Act to revise, consolidate, and amend the Statutes relating to the Post-office Department (17 Stat. 283, enacted June 8, 1872) consolidated the United States Post Office Department into the Cabinet of the United States.
Source: Wikipedia — Post Office Consolidation Act of 1872 (CC BY-SA 4.0)