Bold stamp of WP #351 maker's mark.  It's the Katakana letter "I" encircled ㋑.  Perfect mark with all points showing; a little off center. 

This mark is typically found on 3 1/2" floats, it's an uncommon mark.  But because this maker's mark is on a 2 1/4 inch float, I'm assuming this is a rarer float.