Dag Wim ,Hello Mark I know Jan Spruyt 's nursery and his form of aster alpinus.Sabine and Suzanne are more full at least from what I saw from pictures in the book of the late german nursery man Karl Foerster
I shoud say they seem to be a bit more than double but not flore pleno.I am looking for them for 23 years now.May be I'll find them before I go and push up the daisies.