Clovnfire - This is not a cancel. During the 1930s and 1940s in the time of World War II, mail was opened by censors and the content was read. Your item was used to seal up the envelope after it was opened, which is why it has a crease right down the middle where it was folded over. They were printed in sheets and perforated.
I do not have an example of your exact censor seal, but I do have this cover in my collection with a similar, but different, censor seal, to help show you how your item was used on cover.
Hope this helps you,