On certain Canon FD-lenses (for example the 50mm f/1.8 S.C.) there is a small metal cylinder sticking out of the focus ring (not the button on the aperture ring). What is the purpose of it?
This pin is required for the “Canon Automatic Tuning” (CAT) system which allows for (semi-) automatic flash exposure with the Canon FTb, F-1 and EF.
The pin drives a variable resistor in a “Flash Auto Ring” which is attached to the front of the lens. The focusing distance is electronically transmitted to the Speedlite 133D CAT flash. The flash unit controls the camera's meter needle based on capacitator charge and focussing distance. With the Canon FTb and F-1, lining up the aperture pointer with the needle gives correct flash exposure. In case of the Canon EF, when set to 1/125s and flash mode, the correct aperture is set automatically.
The CAT pin was only found on some 35mm and 50mm breech-lock lenses.
The Canon EX Auto also supports the CAT system with the EX 50mm 1:1.8 lens and the Canolite D flash. The EX Auto is a front component convertible type with thread mount and does not accept FD lenses.
This page has more details on the CAT system and some images.
A number of Leica lenses have a "focusing tab" on them, it lets an experienced user know the focus distance setting by feel - and gives the finger something to hold on to for adjusting the focus as well. Could it be something along those lines? Granted, it wouldn't be as useful on an SLR as on a rangefinder...