You only need the "proper" adapter when you want to use the frame lines in the finder. With the 15mm CV Heliar you'll be using the accessory finder, and you won't be needing the rangefinder because the lens doesn't couple. Those of us using LTM 85mm Nikkor and Canon lenses stick 'em on 90mm adapters. It's close enough.
The only thing to watch out for, whether a Leitz or other brand, is that it does bring up the proper frame line. Some may have been modified ~ a couple strokes of a file on one of the bayonet lugs will change a 50mm adapter into a 35mm adapter. But that's good to know in case you want to do it...LOL