I have picked up a couple of CZ covers.
There were no Canal Zone special delivery stamps, so US special delivery stamps were used. Finding them on cover is always a treat.

Registered covers of this time are not particularly scarce, but there were two aspects that drew me to this cover. The first is that there is a registration stub pasted to the cover, which is a bit unusual.
Second, the registration number is unusual, as it is an "official" registration number. This cover was used to send Canal Zone stamps ordered by a customer in the US. the PA in the PA-9 registry number stands for Postal Agency.
This cover also exhibits well the philatelically-sensitive killer cancel adopted by the Canal Zone in the last several years of its operation, the small circle cancel.

At first, I thought this just an attractive commercial cover, but then I realized the stamps were no ordinary prexies. This was mailed five months after the release of Canal Zone Scott 118-9, the only prexie stamps overprinted for use in the Canal Zone. It is hard to say whether this is a philatelic usage or usage by someone who just assumed they would be good for US postage.
