WP #305 is a 'Sa' サ series mark. There are two symbols that create this mark.
The first symbol is the standard 'Sa' サ character with a hash line underneath.
The second symbol is three lines inside of a rectangle. The three lines could be the symbol for river 川 and the rectangle could be the symbol for mouth 口. Of course, this is just a guess.
This float measures 3 inches in diameter.