Section 1438.16 of Title 3, California Code of Regulations, provides that heads of lettuce “of the 2 ½ dozen size when fairly tightly packed in standard container numbers 45A, 45B, or 45F” may be placed in such containers as a bridge pack, with 12 heads in the lower layer and 12 heads in the upper layer with six heads of the same size between such layers. “Two and one-half dozen size” in the bridge pack described in this section, means heads of lettuce of uniform size which are (1) larger than heads of lettuce that can be packed flat with 15 heads to a layer (3 x 5) in standard container numbers 45A, 45B, or 45F and (2) smaller than heads of lettuce that can be packed with two dozen heads in two layers of 12 heads in each layer (3 x 4) in the same container. When determining the size of heads of lettuce as defined in this section the heads shall be fairly tightly packed but not so tight that the lettuce is damaged or injured in such packing.