This 5 inch float has maker's mark WP #320 'lO.  In this example, the mark has been sucked into the sunken seal a little but it's still quite visible.

According to Walt Pich in his book Glass Ball Marks, "At least six of these are in local collections all 6 are 5-inch floats with the mark on the seal button."  In recent years, more of these 5 inch floats with mark WP #320 have been showing up on eBay.  Case-in-point, this float was acquired in 2008.  I also picked up another one of these in the last couple of years.

Found on Hokkaido Island, Japan.