Where notification is effected by registered letter, whether or not with advice of delivery, such letter shall be deemed to be delivered to the addressee on the tenth day following its posting, unless it has failed to reach the addressee or has reached him at a later date; in the event of any dispute, it shall be incumbent on the European Patent Office to establish that the letter has reached its destination or to establish the date on which the letter was delivered to the addressee, as the case may be.