It's 1.5 mm birch aircraft ply. Years ago someone gave me a lot of Finnish birch ply that was going to be a home built airplane, but the project never got off the ground, so to speak.
It becomes an integral part of the coaming as it is filleted and glassed in place. It is used as part of the mold and after the part is built, the rim is glued to it. The method is detailed on the One Ocean Kayaks web site.