This stamp of WP #351 is the large size version of this mark (compared the the smaller stamps on 2 1/2 inch floats).
The WP #351 mark ㋑ is the Katakana letter "I" that has been encircled.
It has been classified as an uncommon mark in Walt Pich's Marks book.
Beachcombed on Kunashir Island, Russia. Kunashir is one of the Kurile Islands located just north of Japan.