This undocumented mark appears to be 'Maru Yoi' 良.  The stamp is deep and clear ... with all the points of the mark showing.

I really like marks that are deeply pressed into the glass.  These deep marks are typically found applied near the edge of the seal.  This mark is about 1 1/4 inches wide or a little bit bigger than a quarter.

One aspects that draws me to this kind of stamp is how far the Yoi 良 symbol is raised up above the surface of the glass.  They are so bold and clear ... it's easy to see them, even from a distance.

This is an extremely rare float with only 3 examples of this mark that are known in collections.

I won this float in an epic battle on eBay.  It was an expensive purchase for sure but I figured it might be a long time until I'd ever have a chance to buy another Maru Yoi float.