17-32mm Helicoid on left, C-Mount to NEX adapter on right.
Note the two threads inside the C-Mount to NEX adapter. The outer most thread has a 55mm thread while the middle thread size is 42mm. You simply screw the helicoid onto the C-Mount to NEX adapter. The one I used was bought from Rainbow Imaging. They sell on eBay as well as Amazon and their own web site. This one is very inexpensive for around $15 or less. Click here to see a sample one they are selling. I am not connected to them but I bought a few adapters from them and always get excellent service. The best part, if you are in the USA or Canada, is that they ship from USA (at least when I bought from from eBay).
Perfect marriage -- Helicoid screwed onto the C-Mount adapter. Very tight fit without lots of space.
Helicoid mounted on the NEX-5.
Helicoid with EL-Nikkor 50mm f2.8.
The helicoid I am using has a 52mm thread on the lens side, instead of the more common M42. I have many odd lenses that have large rear mount and I find 52mm size is easy to find adapter for, since I have LOTs of useless filters I bought from the Outlet Store. I just remove the glass and voila, useful adapter rings! The last pictures shows the EL-Nikkor is mounted on the 52mm filter ring, and in turn, the filter ring is screwed on to the helicoid. I do need to use electrical tape between the EL-Nikkor and 52mm filter ring, since they fit snuggly, but not very secure by themselves.
Hope you find this useful.