The M6, M6ttl, M7 and MP are all available in versions with a 0.85 magnification viewfinder as well. This is useful for shooting longer lenses as it offers an enlarged view. The downside is that the framelines for 35m lenses are more difficult to see and the 28mm lines are absent altogether.
Additionally, Leica and 3rd party companies make screw-in adapters that allow magnification of any viewfinder. These come in 1.25, 1.35, and 1.4 X magnification factors. Combined with the "standard" 0.72 magnification viewfinder, the options range from 0.90 to just over 1.0.
-J.