As a result of the Taft Agreement of December 3, 1904, the Canal Zone Postal Service began issuing the Third Series on Monday, December 12, 1904. The stamps were issued for the same postal purposes as the stamps of the Second Series.
The Third Series is very interesting and presents the Canal Zone specialist with the best opportunity for plating, in this case a particularly fascinating study and a stiff collecting challenge because of the almost endless combinations of Canal Zone overprints, Panama overprints, Panama surcharges, Canal Zone errors, Panama errors, Canal Zone varieties, Panama varieties, and assorted oddities.
The Third Series includes the 1-cent green (Scott No. 9), the 2-cent rose (Scott Nos. 10 and 11), the 5-cent blue (Scott No. 12), the 8-cent on 50-cent bistre brown (Scott Nos. 14, 15, 18, 19, and 20), and the10-cent yellow (Scott No. 13). Although these ten stamps include three different basic Panama issues, they are considered to be only one Canal Zone issue because each was overprinted with the same Canal Zone overprint plate in at least one and often more of its six stages, and usually several of these stamps were overprinted with the same stage(s).
The majority of the Third Series was first issued December 12, 1904, including the first printings of Scott Nos. 9, 10, 12, 14, and the only printing of Scott Nr. 15. The rest of the stamps—Scott Nos. 11, 18, 19 and 20—were issued in late 1905.