All too often Collectors and Restorers are to quick to erase a very important part of the history of the poster. Dario says “Stop don’t do it!” Preserve that poster AND preserve history!
It is certainly a very important part of the poster and should be kept for several reasons. It is a part of the poster. It verifies the authenticity of the poster.
Of course these are just the things that will generate interesting conversations, “what are those strange numbers and text? ” …and you will be able to educate them, they too then will appreciate why you have gone to such great length to restore and preserve this movie’s history the the poster provenance.
Of course if a NSS bleed is in a extremely intrusive spot, like in the actor’s face then yes, that might be a good reason for removal.
I will always discuss the best possible ways to restore and preserve your poster with you before proceeding with any restoration.