Isn’t it true that the annotations of shelfmark 1427 were made before Oxford purchased it?
No. This is a rumor started by the Folger Shakespeare Library, in a 1993 pamphlet, Roasting the Swan of Avon, and later perpetuated by the Smithsonian Magazine. Despite being promulgated by such supposed authorities as the Folger library and Smithsonian, the argument, as Mark K. Anderson and Roger Stritmatter demonstrated in 1996, was based on erroneous facts and indefensible reasoning.