Section 5A. No agreement to make a will of real or personal property or codicil thereto or to make a bequest or devise, or to revoke or not to revoke a will, codicil, bequest or devise, or to refrain from making a will, codicil, bequest or devise or any other agreement relative to making or not making a will, codicil, bequest or devise, shall be binding unless such agreement is in writing and signed by the person whose executor or administrator is sought to be charged, or by some person duly authorized thereunto by him in writing. This section shall not apply to any agreement made prior to October first, nineteen hundred and sixty-five.