Mikiko Lockwood in an article on The Hosta Library titled, A Little About Japanese Hosta Terms defines the term junpakuka or jyunpaku-ka as pure white flower.

This cultivar name was mentioned in the Lockwood article with no further details but we can probably expect that it is a plant with pure white colored flowers. As a sport of the species Hosta hypoleuca, it should be a medium size hosta with long leaves that come to a point at the tip. The petioles would have reddish purple dots and it would bloom from late July into August.