Hi Robert
Have a look here:
http://www.sciencecenter.net/hutech/...lk/focuslk.htm
Like all things from the USA, it's a bit expensive for a simple mechanical device that a skilled metal worker could make at home if they had the skills and gear.
Allegedly, it appears to reduce any mirror shift to between 1/2 or 1/3, which seems a worthwhile gain when trying to optimize your overall system.
Cheers
Dennis