Bob, I'm definitely not an expert in this area, but my understanding is this:
The book in question has minor changes (you mentioned changing obselete 'f' to 's'). This suggests the more recent edition has been re-typeset. In that case, whoever issued the new edition cannot claim copyright in the actual content of the book, but they do have copyright of the typesetting. If you scan that typesetting, you are infringing their copyright.
There are plenty of folk in this forum are knowledgeable on this subject, so if anyone contradicts the above, chances are that they are right and I am wrong.