“U.S. Combination” consists of a combination of U.S. No. 1 and U.S. No. 2 grapefruit: Provided, That at least 55 percent, by count, meet the requirements of U.S. No. 1 grade for defects, And provided further, That the lot meets the basic requirement for discoloration as specified in the U.S. No. 2 grade. For tolerances see § 51.628.