I was surprised about the Schultheis nursery saying having received this gallica instead of 'Sissinghurst Castle' and more by the fact they tell on their website that this rose couldn't be successfully identified by them. Thus in front of such a beautiful rose, and its commercial interest, they gave the new name 'Fürstin Leonille' in 2012. I think they didn't really search seriously amongst the known gallicas in collections.
This rose is similar, or strictly identifical I should say, to the famous 'L'Hybride de La Maître Ecole', bred in Angers by M. Coquereau in 1831.
This mystery is now solved.