I got three cheap adapters from Amazon vendors when I bought my NEX-5 a couple of years ago (the adapters were for Nikon, Canon FD and Leica screwmount lenses). All three worked on the NEX-5, and so I thought, why waste your money on more expensive adapters?
Then recently I purchased a NEX-VG30 camcorder and - wait a minute! - the one for Leica 39 works fine, but the other two wouldn't mount on the camcorder. I think I may look for more finely machined rings now.
For your information, the two adapters that won't mount both have the "Fotasy" name on them. The Leica screwmount one bears no manufacturer's name.