From Cameraquest,

Voigtlander TYPE II 28/90, 35/135, 50/75 Leica Screw Mount Adapters

$59 Introduced Sept 2007. Type II adapters were introduced to allow bar coding for Leica M8/8.2/9 M cameras (adapters not sold with bar coding). Type II adapters also work some infinity lock lenses IF the infinity lock is set back from the end of the lens barrel -- like the Leitz 50/1.5 Summarit.

Voigtlander Type I 50/75 Leica Screw Mount Adapter

$69 50/75 Type I adapters can NOT be bar coded for digital Leicas, BUT work well on older lenses which have infinity locks located at the back of the lens barrel near the camera body. Type II adapters usually don't mount on lenses with infinity locks, that is why you use Type I. Type I adapters can NOT be bar coded for digital Leicas like the Leica M8/8.2/9.

Actually, I didn't really read it carefully. It seems that type I is ideal but type II doesn't work for some lenses. I'm not sure if they work for the ones I'm interested in mounting. The Type I, I believe, are discontinued and more difficult to find. Maybe I should just pay for a Leica one and assume that I'll have no issues.