This simple bold mark is the Katakana symbol 'Su' ス. It's an undocumented mark.
This unique Su ス mark is the only example known to exist in collections. I think this is an extremely rare float that needs to be preserved for history.
The float itself measures 14 inches in diameter.
This float has excellent provenance. It was found by Kala & Aragon Ellwanger in the Noto Peninsula in Japan.