The lens adapter is required to allow L Mount (Screw Mount) lenses to be fitted on M Bayonet Mount camera bodies.
The M/L adapter comes in 28/90, 35/135, 50/75 combinations. For Voigtlander Bessa cameras, there is no difference which on M/L ring to use.
But for Leica cameras and Zeiss Ikon, (these cameras has automatic finder frame line change system), M/L ring 35/135mm is to be used when you attach 35/135mm lenses on Leica/Zeiss Ikon cameras and M/L 50/75mm is to be used when you attach 50/75mm lenses on Leica/Zeiss Ikon cameras.
This is to allow Leica/Zeiss Ikon cameras to change its frame lines correctly.