424. The beneficiary governed by article 1029 of the Civil Code of Lower Canada and designated before 20 October 1976 is a revocable beneficiary within the meaning of this Act, except(a) the person designated irrevocably by a stipulation to that effect in the policy or in the document effecting the appointment;
(b) the person designated under a contract in which the policyholder or participant has not reserved for himself the right of revocation if this beneficiary has notified in writing to the insurer, before 20 October 1976 or within 12 months after that date but before his revocation, notice of his intention to accept the stipulation in his favour.