This is Silivan (Silvia descendant)
Pavel likes it because of contast edging of the lower petals

It looks like that this variety is non-stop bloomer.
Also the color of the lower three petals is pure bright white (does not have any yellow or pink in the backgound)

Flowers are not large ( about 7cm)
Important that the flowers are well opened which is not always the case with silivans.
Peduncles are not long. Pigment is very stable.

Many silivans' blooms are not stable and fell of easy. This one has double-flowerness gene and that is why blooms stay for a long time.
People belived that they saw something similar among polish streps.
They think that it in some ways similar to Yana
I will try to find a photo to compare.. . .