 On August 15, 1939 the Canal Zone Post Office issued a series of 16 stamps commemorating the 25th anniversary of the canal's completion, depicting "before" and "after" views of various points along the canal.Thereafter stamps appeared at an average rate of about two per year, with a commemorative set in some years and no stamps in others.The inscriptions were changed to just "CANAL ZONE" in the 1960s.