(2) Where the disclosure was made at an exhibition, the applicant shall also file a duly authenticated certificate issued by the authority or person responsible for the exhibition. (3) A certificate under paragraph (2) shall contain particulars of the exhibition together with a statement to the effect that the invention was in fact exhibited there. 20. (1) A declaration claiming priority under section 16 of the Act shall be made at the time of filing the application for the patent and shall indicate - (a) the date of filing of the earlier application; (b) the reference number of the earlier application; (c) the symbol of the International Patent Classification which has been allocated to the earlier application; (d) the country in which the earlier application was filed or, where the earlier application is a regional or an international application, the country or countries for which it was filed; and (e) where the earlier application is a regional or an international application, the national industrial property office of the country with which it was filed. (2) Where at the time of filing the priority declaration the reference number of the earlier application is unknown, the reference number shall be furnished not later than 16 months after the priority date. (3) Where a symbol of the International Patent Classification has not been allocated to the earlier application or had not yet been allocated at the time of filing the priority declaration, the applicant shall state this fact in the declaration and he shall communicate such symbol as soon as it is allocated.