Hello
In my opinion, the Neil Armstrong signed Belgian stamp block posted by decrynne appears to be authentic. These are referred to as Eiermann souvenir sheets and a number of astronauts contracted to sign them commercially in 1969. In many cases these are authentic.
However, unsigned sheets exist, and forgers have used them. The example in my Neil Armstrong Autograph Study is an example of a forgery on the same block. (Thank you johnsim03 for linking to my study!)
I hope this helps.
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