Hi Steve,
I don't know but you are most likely right. I have read everything about San Diego wreck and conducted extensive research on shipwreck tsuba, as I’ve encountered them(iron) before. - But thanks for sharing.
Wanted to share a swordsmith tsuba(Igo no kami Kinmichi) that was once covered with urushi. Luckily, we can still see some remnants of the old lacquer.