Unless my memory is playing tricks on me, this shell comes from the Japanese shell tannery. I have never seen a Horween that size but if someone else has, please correct me.
I would guess that the belt is mostly shell, but there are probably areas there that Horween would dismiss as actually being shell. So basically it is not cut and trimmed to just get the pure shell.
Ron Rider did some sort of experiment with that Japanese Tannery's shell and Horween. He like folded one shell each into like a bowl and filled them with water. Apparently after a bit the water soaked the leather and began dripping through on the Japanese Tanned hide, but not the Horween hide.