This 'Shi' シ mark is quite interesting.  All three strokes begin inline (i.e. align to a single line).  This may be consistent with an "old style" katakana script of the Shi symbol.

This may be an older pre-WWII float.

It's a nice green color.

This float has excellent provenance. It was beachcombed on Kunashir Island in Russia by Olga & Dmitry Sokova.