I forgot to mention: they also sell Maclura Pomifera which I ordered too; quote:
“Wood highly valued for making hunting bows. Large green fruit similar to an orange but not very tasty. Wood is slow to decay in the soil. Used as a root stock for the genus Morus.”
I never knew about Maclura Pomifera before!!! I found this page by googling “morus site:www.treeshrubseeds.com” and finding all “morus” pages on their site; they have for example pages with “morus macroura” (dried fruits; white and black) but some site problems, you cannot order it
I googled Maclura Prolifera and found that it is valued as rootstock because mulberries will grow more upright.
here in Canada, nurseries usually prefer Morus Alba Tatarica as rootstock, because of cold hardiness (better than Morus Alba); although Morus Alba hybrids grows everywhere as weed, even listed as invasive species.
P.S. I googled it again, “Osage-Orange”, back in Azerbaijan it had a name “Monkey’s Bread”; I don’t think it is good rootstock for Mulberry (Morus Nigra specifically), otherwise we would know already 