AO-U546-1,1964, New York Worlds Fair, First Day Cover, Add-on Cachet, 5 cent, postal stationary 

Add-on cachet is:

A cachet design added to a cover which was originally uncacheted. An add-on cachet should be identified

by maker and date (mine are stamped on the back with the date) so that it is clear that it is not contemporary with the cover.