I bought a couple of these online very recently from Nature Hills Nursery; probably should have called them, but just tacked them impulsively onto another order. They were advertised as Juglans nigra var. Carpathian, with a description focusing on cold hardiness and nuts being the English (i.e., Persian) thin-shelled type. I assumed the nursery had stated the species in error and that it was in fact Juglans regia var. Carpathian. I have seen this variety referred to many times in the past, whereas I've never heard of a Carpathian variety of the black walnut.
Later, a friend told me he had heard of a Carpathian walnut that is a hybrid of J. nigra & J. regia, and that is what it probably is, but I can't find any reference to such a thing anywhere in books or online. When the leaves come out it will tell me something, because apparently J. regia always has 9 or fewer leaflets, J. nigra 11 or more. But if it is a hybrid, this would not be a foolproof way to identify it, would it?