Section 134A. If any individual insured under a group life insurance policy hereafter issued becomes entitled under the terms of such policy to convert to another type of life insurance within a specified time after the happening of an event, such certificate-holder shall be notified of such privilege and its duration within fifteen days after the happening of the event; provided, that if such notice be given more than fifteen days, but less than ninety days after the happening of such event, the time allowed for the exercise of such privilege of conversion shall be extended for fifteen days after the giving of such notice. If such notice be not given within ninety days after the happening of the event, the time allowed for the exercise of such conversion privilege shall expire at the end of such ninety days. Written notice by the employer given to the certificate-holder or mailed to the certificate-holder at his last known address, or written notice by the insurer mailed to the certificate-holder at the last address furnished to the insurer by the employer, shall be deemed full compliance with the provisions of this subdivision for the giving of notice.