All deeds or other writings executed by an agent or attorney-in-fact for a person in the armed forces or military service of the United States, or for a person who after executing a power of attorney or agency agreement entered the armed forces or military service of the United States, or for a person who departed from the United States by permission or direction of any department or official of the United States in connection with work relating to the prosecution of the war, when the power of attorney or agency agreement under which the deed or other instrument was signed was not executed in such a manner as to be valid as a sealed instrument, shall be held, and the same are hereby declared, valid and effective in all respects if otherwise valid according to the law then in force. The provisions of this section shall not operate to affect adversely intervening vested rights.