Here are some recently described Records of the Post Office Department (Record Group 28). Further information can be found in the National Archives Catalog.
Records of Postal Stations and Branches at Pittsburgh, Pennsylvania, ca. 1910 – 1952 (NAID 12166787).
Efficiency Reports Concerning City Letter Carriers, 4/1920 – 5/1920 (NAID 12093015) .
Records Concerning City Delivery Allowances, 1912 – 1919 (NAID 12087690).