select distinct paper from paper_master,np_master,stamps where stamps.paper_code=paper_master.paper_code and paper_master.paper<>' '  and year(stamps.issue_date)=1977 and month(issue_date)=4  and np_master.np_code=stamps.np_code and np_master.np=\'P\'  and stamps.serial_no=\'0847\'  order by paper