Currently there are two firms that modify Contax G lenses to Leica M. Hawk (Taiwan) whom I represent, and MS Optical is the other (Japan.)
The conversion as shown in the photos above takes an original G lens, removes the rear part of the lens barrel (including the Contax bayonet, autofocusing gearing and AF drive screw) and replaces it with a standard helical, a coupled rangefinder cam, and Leica M bayonet. The optics module (the glass) and the front part of the lens (aperture ring, filter thread, and retaining ring with the Zeiss text) remain as they were originally and are not disassembled. The original leftover parts can be retained in case the owner wants to reverse the process.
The MS Optical conversion as listed on the Japan Exposures website completely disassembles the optics from the original barrel, which is then discarded. The individual glass elements are then remounted in a different barrel, with a helical, RF cam, and aperture ring.
The MS Optical conversion has rather challenging ergonomics similar to old Leica screw mount lenses. The aperture ring has no click stops, for example, and will rotate when the lens is focused. The apertures appear in the reverse order from the original lenses. Ken Rockwell has done a review of the MS Optical conversions and gives more detail.
The Hawk conversion is a more 'normal' feeling lens. The aperture ring is the same as it was on the Contax G cameras...it doesn't rotate when the lens focuses, and the helical has a good feel and grip.
Several getdpi members have used the prototype 45/2, it's gone from Washington to Arizona to California to NYC to Hawaii, and every seems to really like it.
Here are some pics that Ashwin took with it:
http://bit.ly/zXnhzr
And more here:
http://bit.ly/KZ05No
Feel free to contact me, email is best: robert.jagitsch (at) gmail.com