Any instrument in writing executed by an attorney-in-fact shall be good and valid as the instrument of the principal, whether or not said instrument is signed and/or acknowledged in the name of the principal by the attorney-in-fact or by the attorney-in-fact designating himself as attorney-in-fact for the principal or acknowledged in the name of the attorney-in-fact without naming the principal from which it will appear that it was the purpose of the attorney-in-fact to be acting for and on behalf of the principal mentioned or referred to in the instrument. This section shall not affect any pending litigation or the status of any matter heretofore determined by the courts. This section shall apply to all such instruments heretofore or hereafter executed. Registers of deeds shall be required to index all such instruments filed for registration both in the name of the principal or principals executing the powers of appointment and in the name of the attorney-in-fact executing the instrument: Provided, that instruments heretofore registered and indexed only in the name of the attorney-in-fact shall be valid and in all respects binding upon the principal or principals insofar as validity of registration is concerned. (1945, c. 204; 1959, c. 210.)