There's a lightening hole in the center and the curve on the back side was cut with the bandsaw and smoothed with the spindle sander.
Here they are during construction.
The seats are held together with two 5/8 in thick rails 2 inches wide.
The rails are beveled on the sides and cut to the curve at the back.
Here they are in place.
The seat backs are made from 3003 aluminiun .032 in. thick. One of the nice things about being married to an ex-school teacher is that I get to use her leftover pink poster board to make the radius templates! (before you make fun of me).