Work continues on the seatbacks. Remaining to be done was the bottom side attach hinge that attaches the seat back to the hinges we previously installed on the seat floors.

Vans made this funky design that makes you grind away some material from the aluminum angle and squeezes the hinge between angle and skin. On the back side, an additional angle is installed for strength.
This is what it should look like. Takes a bit of time to grind away the material on all 4, but it's a straightforward task.

The bottom side of the seat back is where the left and right seat are different (it's actually the only difference, because for the rest they are identical twins).

The place where the hinge starts with an eye and ends on the other side without eye, is the inboard side of the plane.
So for the left seat (looking from the firewall to the back) the eye is on the left side. For the right seat, the eye is on the right side

Now you have to drill the hinge to the seat back skin and angle.
The dimensions are kind of weird in the plans. They want you to drill on a 5/16" line measured from the bottom of the hinge.
Doing so will leave little or no edge distance left for #30 holes on the hinge. Now ok, it's squeezed in between the angles on the sides, but the middle would have very little edge distance.

I really can't see a reason not to put this line a bit more toward the middle so I compromised between the center line and the 5/16" line.
If something breaks, I perfer it's be the back supporting angle  instead of the hinge.
Now drill all holes on the hinge but leave the side angles still unattached.For the distance of the side of the hinge, I measured first from the side of the side angle to the center of the holes and copied that on the hinge.
Once drilled, cleco the side angles back on.

Another shot of the lower part.

Since you drilled already the side holes on the seatback skin, hinge and back angle, it's now easy just to backdrill the side angle from the back.
As you can see on the back angle, I still have enough edge distance on the angle.

Enlarged holes to #40.

After this, don't forget to cut back the center eye from the bottom hinge as there will be a left and right hinge pin entering there.

Then cut off the corner of the side angle for all the bottom locations.

For the top, they want you to be more fancy and make a nice rounded top on the angle.

 Finally, I temporarily installed both seats on the floor with some hinges.

 Another shot from the side Ain't that incredibly cool.

I may leave te cleco's in. I heared accupuncture is quite good for the back.