Railway Post Office mail sack label
This wooden pouch tag, circa 1870, predates paper labels that were approximately one-by-four inches. It was attached to a pouch by a lock's hasp that passed through the slot in the tag.
This tag covered dispatches made by the Sheboygan and Princeton RPO to the Green Bay and Milwaukee RPO. Both of these routes operated over Chicago & Northwestern Railway lines. By the 1880s, the Green Bay and Milwaukee route was renamed the Ishpeming and Chicago RPO. "Plymouth So." refers to the mail separation contained in the dispatch. Mail for Post Offices south of Plymouth along the Green Bay and Milwaukee RPO was enclosed.
A disadvantage of wooden pouch tags was that they had to be returned to the origin Railway Post Office (RPO) crew after each use. RPOs had to sort them and keep a reserve on hand for dispatches.