Hello Marc, again.
I have read your original post along with the various responses from other forum members. Here's the rangefinder story.
Internal adjustments that MUST be performed with top plate removed - the parallax frame line adjustment.
Horizontal RF adjustment - can be done without dissembly and through the lens mount opening.
Vertical RF adjustment - M6: done through 'access opening' hidden behind the red Leica logo on the front of the camera. Other RF Ms: this opening is covered with a screw.
If you screwed up the adjustments, you MAY damage the mechanical parts involved or cosmetically damage the camera's appearance - so be warned about this BEFORE you undertake this!
DON'T exert too much force when you make any of these adjustments AND each individual adjustment MAY affect the other adjustments - be forewarned!!!
There are a total of four adjusts that is accessible through the lens mount opening. Two affect the (optical as seen through the VF) horizontal alignment at two test settings - 1m and infinity. The remaining two are mechanical which affect the limit of arm travel and alignment of roller to the lens cam.
Horizontal adjustment is as several posts have indicated, can be adjusted through the lens mount opening.
1.) on the rangefinder actuating arm, there is a follower roller which mates with the focusing cam on the M lenses. The 'pivot' screw is actually an eccentric pivot AND is adjustable. That's used for the 'infinity' adjustment.
It's best to start off with a 1/4 inch flat-bladed screw driver that has a five or six inch tang. You'll have to grind the tip to approx. 5mm as this pivot adjustment screw has a 5mm slot. You may also have to grind the tang diameter down a bit at the point where the tang 'touches' the lower portion of the lens mount (6 o'clock position). With this modified tool you can touch up the horizontal alignment for infinity - it's a 'remove lens, adjust, re-mount lens, test, and repeat' process. Caution, apply too much force on the screw WILL distort the arm and you will have problems with the follower roller mating the lens cam properly.
The second adjustment affects accuracy of the RF a 1m. Adjustment of this MAY also affect the infinity adjustment as well. This second eccentric is part of the MAIN arm pivot 'assembly.' Under 'normal camera use,' you'll RARELY need to touch this at all!
I'm not going to discuss the two remaining 'mechanical' adjustments that may be done through the lens mount opening.
M6 vertical RF alignment - this is Marc's main concern. On all other M RF bodies, the adjustment point is behind the access hole covered by a screw. On the M6, this opening is hidden by the round decorative red Leica logo disk.
Removal of this disk IS necessary. It is 'glued' on. Some people can remove it without damage, most will probably damage it upon its removal and in the process MAY also scratch the surrounding areas!!!!!! Exercise caution here.
On previous M RF bodies, you need a small flat-bladed jeweller's screwdriver. The adjustment point is a tiny slotted screw on the other side of the access hole.
On the M6, Leica CHANGED the mechanicals - you need a very expensive, for the lack of the correct term, 'special' screw driver to alter this adjustment. This is the tool I described in my first post on this topic.
After this veritcal adjustment is done, you'll need to re-attach the Leica logo disk or buy a new one as the original may have been damaged.
I won't discuss the internal adjustments AS you'll need a set of highly customized tools JUST to remove the top plate AND then you'll need the test targets before you can handle these adjustments. With routine camera use, these internal adjustments are NOT necessary!
I hope this settles the RF adjustment controversy. Just beware, adjusters heed my earlier warnings!