I beg to differ with you ratio411, the stamp itself was printed in 1939 then overprinted in 1944. I'd say that is close enough to call it classic. Also the stamp is listed in the US section of the Scott Specialized Catalog, and does say United States on it.

That being said, I do
not believe the overprint is genuine. I am not an expert but the top bar in the letter T is too thick and lopsided. Also, the space between the C and T should be more pronounced that the spacing between the other letters.
It is most likely a forgery. The base stamp is quite common, and the overprinted stamp, as a mint stamp catalogs over $1000. Given that it varies obviously from the catalog description,
I would not even recommend having it expertised unless it was to be able to return the stamp as a forgery.