This float with mark WP #378 'Ne' 根 is distinctive and rare.

First, the stamp is the type that has been deeply pressed into the glass on the edge of the seal button.  The stamp is about 1 inch wide.

The mark appears to be the Japanese symbol 'Ne' 根.  It translates to mean "root, source, or origin".  It could also be used as a surrogate for a family name, such as Nemuro, for example.

Second, it is very rare.  Two floats with this mark were found and photographed by Walt Pich in Japan in the late 1990's or early 2000's.  Both were 14 inch floats.  Walt left these floats Japan hoping to bring one back on later trip but they disappeared before Walt could retrieve one.  

This float measures 44 1/2 inches in circumference or about 14 inches in diameter.

I purchased this float in Japan in April of 2023.  As far as I know, this is the only example of mark WP #378 back here in the States.