AGRICULTURE - COMMON ORGANIZATION OF THE MARKETS - MILK AND MILK PRODUCTS - EXPORT CERTIFICATES WITH ADVANCE FIXING OF REFUNDS - DAY OF ISSUE - CONCEPT - EXPORTS IN CONNECTION WITH INVITATIONS TO TENDER OPENED IN AN IMPORTING NON-MEMBER COUNTRY - DATE OF ISSUE - CONCEPT ( REGULATION NO 2044/75 OF THE COMMISSION , ARTS 3 ( 3 ) AND 6 )