Within twenty days after the service of any such demand upon any person, or at any time before the return date specified in the demand, whichever period is shorter, or within such period exceeding twenty days after service or in excess of such return date as may be prescribed in writing, subsequent to service, by any antitrust investigator named in the demand, such person may file and serve upon such antitrust investigator, and in the case of any express demand for any product of discovery upon the person from whom such discovery was obtained, a petition for an order modifying or setting aside such demand—