So here is what I have:
Momo horn ring with holes clipped out. Momo pattern is 70mm where Nardi is 74mm:
Here is the OD of the area of the Momo horn button that snaps into the ring:
OD of the area on the Nardi button that snaps into the ring... as you can see there is a small difference:
There is a small lip around the top edge of the buttons. Here is the Momo OD on that lip:
Here is the OD on the Nardi lip:
The lip on the Nardi horn button is just wide enough to sit on top of the ring which hides all the screws (shown on the right)...
With the Momo, its too small and the ring just falls off. Ill probably secure it somehow... Im not too worried about it. I was just cleaning up the living room and found the girlfriends hot glue gun. I may actually just glue the ring on. Its very secure, but since its metal on metal, I can peel them apart if I really need to with no damage being done.
Overall the button was a success. I like it far more than the stock Nadi design. Someone needs to make blank Nardi/Momo buttons where you can insert your own designs with a cap on them so it looks factory.