For sale is a Volvo servo clutch (P/N 1671953). Never used. Made in the USA.
This can be used as a factory replacement or some people have added these to their hydraulic clutch to convert to an air/hydraulic clutch. This makes the clutch much easier to push (see here). That was my plan if I had bought an HDT with a manual transmission or Autoshift. However, I ended up with a clutchless automated transmission.
Asking $240 (this is what I paid for it). Includes shipping.