This undocumented mark is comprised of the two symbols Shi シ and Zu ズ.
This is an extremely rare float because it's the only example of the Shi Zu mark known to exist in collections.
This is a large float measuring 47 3/4 inches around or about 15 inches wide.
It has excellent provenance having been found in the Noto Peninsula area of Japan by Kala & Aragon Ellwanger in 2021.