While removing the cushion I also found Katakana writing on the wooden backing. The writing is hard to see (pencil on wood).
I can make out: バック レスト(Bakku resuto, "back rest")
Our boat was manufactured in Japan(Yamaha), and we keep finding Japanese writing in the strangests of places.
@rek All we had were German markings, but I left a message on a repair job I did that is hard to find. No message of discovery from the new owner yet ;-)