Thanks for reading, and for pointing out the errors with the filenames. I’ll also add the compiled contracts to Github this afternoon if you think they’ll be useful to people.
As for your error, somehow it seems you’ve got a non-modified version of the interface file. If you change the visibility of that function declaration from external to public, then it should jive with my contracts. There are a few other points where I’ve changed visibility and mutability of functions in the interfaces, but it’s all in accordance with the standard.