I happened to see this stamp online, and didn't recognize the overprint. It is part of small lot of 8 different Canal Zone stamps on a stamp-selling site, but this is the only stamp in the lot with that overprint style.
I looked through my 2012 Scott US Specialized Catalogues's Canal Zone section, but I don't see that overprint style, and I don't see that that stamp issue was used for Canal Zone.
Can anyone explain it? Is it fake, a provisional, or something else? I saw it and was curious. I just now notice that the part of the wavy line cancel looks drawn in...