It's known as the "inscription block" and appears on most all full
sheets from that period in the selvage. "Use Zip Code" and "Mail Early
In The Day" with the illustration of Mister Zip blocks are common
although with respect to the quantity printed ...
Have it expertised by the Philatelic Foundation.
http://www.philatelicfoundation.org/ or any distinquished philatelic
expertising committee and end the confusion.
You can download a .PDF copy of the latest Postal Operations Manual
here:
http://www.apwu.org/sites/apwu/files/resource-files/POM%209%20Postal%20Operations%20Manual%20%5Bupdated%20through%2010-13%5D.pdf
Go to Section 2 "Philately" and find Subsection...
