Picture 1 shows the left Bulkhead Side Assembly riveted to the seat floor, followed by addition of a Bulkhead Cap (the Z shaped piece). Same for the other side (not shown).
Each of these bulkhead "towers" has a roller fitted. The one shown in the second picture has the roller at the rear of the slot. The spar on the left wing is further back than the right wing so the bottom of the spar slides on this roller. There is a plastic retainer block higher up the tower to guide the top of the spar.
The right wing has the roller and the retainer block mounted on the forward wall as the spar for this wing is in front of the left wing.
Note: Later on I discovered the rollers were the wrong way around. Follow the plans!
