We know the stamp says “USPO” after they had changed their name to “USPS”
Somebody must have believed they actually meant something ~ but all they meant was IF the dater was set correctly somebody touched that piece of paper on that day. Or, the other meaning IF the dater was not set correctly we don't know what date somebody touched that piece of paper.
The proper dating by round dater was not highly regulated! People changed the dates because they loved to do that ~ rather than work! The system was more akin to a church service than creation of an evidentiary trail.