501. No person shall, in any port in which there is a person practising the profession of a Surveyor of ships and holding a certificate granted under section 500, practice such profession in such port unless he holds a certificate granted under that section: Provided that nothing herein contained shall prevent any person employed by a classification society which, the Government may, by notification in the official Gazette, specify in this behalf, from discharging any of the duties of such employment.