There is an image of this cultivar on the Hosta Library. From the photo on the HL website, it appears to be a medium size hosta with dark green leaves and an irregular yellow medial (center) variegation. The petioles have red dots. This non-registered cultivar was originated in Japan by Hisashi. 

Mikiko Lockwood in an article on The Hosta Library titled, A Little About Japanese Hosta Terms defines the term iwa as rock, Iwa Gibōshi or H. longipes and the term nakafu as medio-variegated.