Given the size It is not a Buttercup and not a Black Futtsu either,Yokohamo is described as gray,it is a relative of Shireman both Japanese and similar but too big if it is still growing..
I think this may be a wait and see one, the colour should change as it matures.
I could identify the family by stem and leaf shape and texture if you wish which would narrow it down a bit but frankly right now I can't place it.
Maybe it is just an odd ball one from another country that I don't know. I certainly don't know the name which is unusual and I can't recognise the company logo on the packet either.
Keep in touch Tim, you have perked my interest,,save seeds..
XX Jeannine
Tim just read your thread again, is the 10 inches all around it , how wide is it.Buttercups grow to about 5 inches, but that wouldn't explain the name.??