3 of the pieces are used on their sides to shape the longerons...the top wooden hardpoints that run the length and define the top of the fuselage up to the bottom of the canopy. Nothing really different from the plans here but be sure to read and understand the Cozy FAQs for Ch 5 on the Unofficial Cozy Builders Web Site. You should do this for every chapter before beginning. There is some key info there about the overhang for the longerons that will help make things all come together well in Ch 6.
Here are pictures of the jig ready to shape the longerons (left) and of the longerons after epoxying them into shape (held in place with nailed blocks), floxing (epoxy with flocked cotton added for structure) in the doublers in three places, and curing (right).
Below are the finished products ready to be glassed into the fuselage sides. The boys hopped in to demonstrate size.