I loved building this great kit from Mr. Pilsworth! I think the flat pieces without tabs close off the wing support box structure, and the ribs (parts 16b~d) are installed on the marked sides of the wing spar (the sides that have the partial fuselage cross-section outlines). Here’s an original post from Gary from a few years ago showing how the supports are installed, hope this helps. Great build, really excited to see it completed!
https://www.papermodelers.com/forum/542641-post33.html