This appears to be mark WP #370.  It looks like this could be a Nu ヌ symbol with Ichi 一 underneath it.

This float measures 8 5/8 inches in circumference or about 2 3/4 inches in diameter.  I've seen other examples of the mark in the 2 3/4 inch diameter size.

This float originally came from Japan.  It was in a large container full of floats that Michelle & Shane Konkle brought over from Japan in 2021.