Recently a sport of Alba Semi-Plena is discovered on a branch of Rosa cannabifolia (at nursery De Bierkreek in the Netherlands). As Rosa cannabifola is a real oddity, we can asume that by the malformations (the foliage does not resemble that of a rose) that Rosa cannabifolia is the sport and Alba Semi-Plena the original plant.
See a picture and comment on my website at Rosa Alba Maxima: http://oldroses.nl/my-roses/albas/alba-maxima/