(a) Combinations of the above grades may be used as follows:

(1) Combination U.S. Extra Fancy and U.S. Fancy;

(2) Combination U.S. Fancy and U.S. No. 1; and

(3) Combination U.S. No. 1 and U.S. Utility.

(b) Combinations other than these are not permitted in connection with the U.S. apple grades. When Combination grades are packed, at least 50 percent of the apples in any lot shall meet the requirements of the higher grade in the combination. (See §51.306.)