There is an image of this
cultivar on the
Hosta Library. From the photo on the HL website, it appears to be a
size hosta with green leaves and yellow medial
(center) variegation. Some of the lanceolate, pointed
leaves twist to the side and are quite distorted. This non-registered cultivar was
originated in Japan.
Mikiko Lockwood in an article on The Hosta Library titled,
A Little About Japanese Hosta Terms defines the term iwa as rock, Iwa Giboshi or
H. longipes and the term shironakafu or shiro nakafu as white-center(ed).