Having built Prototype #3, I continued to experiment with and tweak the design.
Routed keyhole in underside of seat slat
Keyhole with bolt inserted
Seat slats bolted to front stretcher
1/8″ strip of wood keeps back slats in alignment
Some various bolt heads
More bolt heads
1/8″ strip of ash in bottom of back slats
Cutouts in seat slats, stretcher, and recesses in leg
I remade the back slats to experiment with the bottom edge; these are short because they are offcuts from the old back slats
Experimenting with different back slat designs