Stephen Gandy has some adapters made that work with Leica M mount rangefinders, and take Nikon F/AI/AIS/AF, Leica R, Contax/Yashica (RTS), Olympus OM, Pentax K & M42 Lenses. There's a cam on the adapter that you use for rangefinding, then transfer that reading to the scale on the SLR lens. Best at further distances and smaller apertures I'd guess, but they focus to infinity. The rangefinding scale on the adapter looks like it's "expanded" more than a normal lens, to get somewhat better accuracy on the readout.
Originally Posted by GaussianNoise
Could that be what you heard about?