This is a Canal Zone Postal Service new issue announcement card; these were mailed to collectors on their mailing list from the late 1930s to 1979, and this one from May 1976 announces CZ C51 and C53, the 22c and 35c airmails issued to cover the airmail rate to South America and the rest of the world. HOWEVER, not only is the card unaddressed (probably mailed out with orders for CZ stamps) but it has a CZ Postal Service rubberstamp cachet used for the issue on the front. Unique? VF and no faults. A great addition to a specialist Canal Zone collection.