This is an undocumented mark with similarities to both mark WP #295 and WP #296. It looks like a mirror image of mark WP #295.
It is 'Seihokei Ma' which is the katakana 'ma' symbol マ inside of a box.
The mark is a bold stamp that completely fills the seal.
This float is 3 inches in diameter and has lots of bubbles.
Beachcombed on Kunashir Island, Russia. Kunashir Island is one of the four major islands in the Kurile Islands located north of Japan.
Olga & Dmitry found this float. Olga sent the following story about their discovery.
Olga: "This float was found five years ago not far from the Yujno-Kurilsk village. After one huge storm the sea shore was deformed and we found an old japanese sheld, the ocean washed it up. Under the wall of it we found a lot of glass floats, the most part of them had this mark."