@PoStat4evR - Just a few clarifications as you seem interested in the history of the Canal Zone.
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The Canal Zone was a US protectorate
The proper term is a US Possession.
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. . . during the building of the canal and afterward until a few years ago . . .
From 1904 till 1980
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. . . when the lease reverted back to Panama . . .
The lease terms were in perpetuity. There were no allowances in the lease for the Canal and Canal Zone to be turned over to Panama.
The US unilaterally ended the lease, and granted all properties, canal operations and canal operating land to Panama. To say that the Panama Canal "reverted" is incorrect as the Panama Canal had always been the property of the USA until it was turned over to Panama.
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Later they printed their own Canal Zone stamps.
Stamps printed expressly for use in the Canal Zone were printed by the US Bureau of Printing and Engraving starting in 1928.
Cheers!