I suspect that Ahrends' "Schnee" primulas are probably all still in existence in gardens, but their identity is long lost.
The two I have are visually indistinguishable, but where I first planted them, 'Schneesturm' thrived and 'Schneetreiben' moped. I moved 'Schneetreiben' to a different location where other P. × pruhoniciana cultivars had done well, and it perked up and has grown quite happily since. This suggests that in favored gardens, these delicate little plants can exist with little or no care.
You, Lesley, may very well be positioned better in NZ to recover some of these "lost" cultivars than those of us who live in less primula-congenial climates.
As a concession to the sensibilities of the readership I will refrain from mentioning another lost cultivar of P. × pruhoniciana, 'E. R. Janes', supposedly orange-colored, and evidently long gone.
The thing I like about these "Schnee" cultivars is the small size of the flowers. I'm very tired of the bigger=better equation; there's plenty of room in the garden for small delights that don't impose themselves on you, though they require some care in siting. Other examples of small=beautiful are Acis autumnale and Anemone caucasica.