This has been done before, but to my knowledge, never so well or so robustly: The hubs are a single piece, machined from bar, and so is the plastic midsection. The hubs can be machined from aluminum, and then anodized (for low friction), or stainless steel. Midsections are available in either Delrin, or a remarkable 30% carbon-fiber-filled PEEK. Quality is outstanding -- all subcomponents are machined in the US, with an eye toward detail.
The axes of the two cylindrical fins actually intersect one another -- they lie in one plane, instead of being offset. The result is a remarkably efficient and smooth flexion -- there's a flash video of one of these couplings rotating at the website.