This is the Signature of Artur da Costa e Silva, so I guess he is the author. If in doubt: Governo do Brasil
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It is the text of a treaty, convention, law, decree, regulation, judicial decision, or other official enactment. (Law 9610/1998, art. 8)
It is a work whose authors' rights belong to the Brazilian government (federal, state, or municipal), for which the economic rights shall be protected for a period of 70 years from the first of January of the year following that of their disclosure or that of the author's demise, whichever is later. ([1])
This signature is believed to be ineligible for copyright and therefore in the public domain because it falls below the required level of originality for copyright protection both in the United States and in the source country (if different). In this case, the source country (e.g. the country of nationality of the signatory) is believed to be Brazil.
Note that this tag cannot be used on all signatures, as not all signatures are copyright-free.