“ Petitioner had instituted an administrative enforcement proceeding against the respondents, a group of individuals and business entities operating as the 'Skycoach' air travel system. The Board's complaint charged violation of its regulations as well as of the Civil Aeronautics Act and sought certain revocation and cease-and-desist orders against respondents. In the course of the proceedings, the Hearing Examiner issued a number of subpoenas duces tecum calling for the production of certain categories of documents of the respondent companies covering specified periods of time. ”