It is not correct compare cameras as Contax II, III, IIa or IIIa to the Leica M5. The Leica M5 have a true TTL meter, simple to use, FULL COUPLED TO THE APERTURE AND SHUTTER DIALS, the same bayonet mount of the M6 or MP, simple and quick to use, and the same rapid shutter advance lever. All the Contax cameras series have a very slow and inconvenient bayonet mount and the button to advance the film. The Contax III and IIIa have an external selenium cell, absolutely not coupled to aperture and shutter dials.
The difference between M5 and M6 or MP are only the needle in the viewfinder (M6 and MP have the LEDs) and the CDS cell (the M6 and MP have the silicon cell). But the M5 show the shutter time in the viewfinder, M6 and MP show only the LEDs. Too little differences to replace the M5 in the preistoric history.
Ciao.
Vincenzo