Section 81V. In case of approval of a plan by action of the planning board, after the expiration of twenty days without notice of appeal to the superior court or the land court, or if appeal has been taken after the entry of a final decree of the court sustaining the approval of such plan, the planning board shall cause to be made upon the plan a written endorsement of its approval. In case of the approval of a plan by reason of the failure of the planning board to act within the time prescribed, the city or town clerk shall, after the expiration of twenty days without notice of appeal to the superior court or the land court, or, if appeal has been taken, after receipt of certified records of the superior court or the land court indicating that such approval has become final, issue a certificate stating the date of the submission of the plan for approval, the fact that the planning board failed to take final action and that the approval resulting from such failure has become final. The plan bearing such endorsement or the plan and such certificate, as the case may be, shall be delivered by the planning board, or, in the case of the certificate, by the city or town clerk, to the person who submitted such plan. Except as provided in section eighty-one E, the existence of an official map in a city or town shall not affect the operation of the subdivision control law therein.