Ed Oliver did say that he usually carries one of each variation in stock, If it's a genuine sorby adaptor, I don't suppose it's that badly priced, like you said making a one off would be a pain. He did say it's a big thing! Can't be an everyday item though!!!
Apparently patriot chucks are available with 1 1/2" x 6 thread. But not 1 1/2" x 8. Typical
And also I now know why the guy I bought my lathe from, was selling his bison chuck separately!!
What a great bloke though, will happily get stuff from him in the future.