Just going by the excellent picture you posted and not seeing the rest of the book I would go by the 2016 Overstreet grading guide : 6.5 would be the best grade I could go based on just that one staple, "Slight to moderate rust migration allowed, even if the staple is rusted, that one doesn't look bad the staple tears also would put it into the same grade. Your question implies that you want to know how rusting works, Often when rusting first occurs it is only in very small dots that sort of crystallize, They fleck off easily but also easily discolor the paper almost like a little piece of chalk, rust first shows on the paper as rust migration and of course can get way worse, The same grade allows for the staple to be replaced with another from a book of the same time but I see no advantage in doing that, the tear and discoloration would not change.