Commission Regulation (EEC) No 646/80 of 17 March 1980 amending for the 11th time Regulation (EEC) No 2044/75 and amending Regulation (EEC) No 210/69 as regards export licences and certificates for butter, butteroil and skimmed-milk powder exported pursuant to Regulations (EEC) No 303/77 and (EEC) No 400/80