The top two characters on this stamp are similar to mark WP #378 but there are other lines and symbols below the top two characters that make this a different mark.
Other differences are that mark WP #378 has only been found on 14" floats and the photo of the mark I saw was of a deep bold stamp.
This float is 5 inches in diameter and the mark is clear but is stamped rather lightly on the the seal.
Found in Northern Japan.
This float has excellent provenance having come from the ex-Col. Rich Richardson collection.