I oversimplified my first response. Here are scans from sample Postal Law & Regulation volumes to illustrate how the procedures evolved over time. It would be an interesting challenge to find examples from each decade or so.
From the "Postal Laws and Regulations of the United States of America, 1847" Wierenga reprint, 1980:


From the PL&R of 1866, only a partial copy as it goes on for several more pages:


And even as late as the 1948 PL&R, again only a partial copy:

Jim Milgram has also written about these advertised markings in The Chronicle in recent years.