Tom Abrahamsson of RapidWinder.comIt is almost never that an inventor improves on a Leica product so that it is better than the original Leica product. Tom holds that distinction with his RapidWinder for Leica M rangefinders -- a bottom mounting baseplate trigger advance. In addition Tom manufacturers other Leica accessories such as his very popular Soft Release and MiniSoftRelease shutter releases. Tom is well known as one of the true Leica rangefinder experts, even by Leica.
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From memory.... it is either to adjust the RF mechanism (the inner adjustment for close distance) or to remove the RF follower arm (when disassembling the camera). To reach the screws easily you can peek a screwdriver through the hole. I am not sure if the camera works without the plug, light might shine through it into the film chamber.
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The metering cell on the bottom of the camera is for the flash TTL metering (my M6TTL has it also).
Not sure what that plug is for though. The plug is still there on the M6TTL, NEXT to the TTL flash meter, and it's there in my M3 DS as well...
If you take the bottom plate off, you can run a long screwdriver up through the hole as Ronald M mentioned. Otherwise, you need to bend a screwdriver at an angle so that it will grip the thread correctly when going through the lens mount. It's easy to strip the screw for the infinity cam adjustment.
As Gabor said, this hole is also the only way to take the RF follower off entirely. Which you need to do when taking the RF off (once the top plate has been removed), and disassemble the camera further.
Not sure the camera is light tight without the plug and lens removed.
As stated, it is to adjust the rangefinder and also to remove the cam-assembly. It should be light tight as there is a coverplate at the bottom + the baseplate. If you are worried, put a piece of black tape over it. You can also get in touch with Leica for the part - esy to install - just match the cut-outs to the "ears" on the plug and with your finger, turn it 90 degrees.