Easiness to create the design
(Conclusion) The registered design was easy to create based on prior arts. (Design right owner lost.)
The shape of the end of the Prior art 1 is formed arbitrarily so as to mate with the opposite parts (the locking rib of the fixing plate).
Therfore, a person skilled in the art in the field of building parts easily change the shape of the end of the Prior art 1 by applying the shape of the end of the Prior art 2 to make the front view hat-shaped and the plan view horizontally rectangular plate-shaped so as to mate with the opposite parts (the locking rib of the fixing plate).
<Writer: Hideki Takaishi (Attorney-at-law licensed in Japan and California)>
https://www.ip.courts.go.jp/app/files/hanrei_en/313/002313.pdf