Vinny,
To test for sure, install the camera and use swing and tilt without the filter to be at the maximum coverage of the lens. This way, you will see when installing the filter/ring combination if the coverage is affected or not.
As Schneider has not standardized filter sizes, I doubt you are not reducing the lens coverage with the step ring. (The Nikon example above is irrelevant. Nikon has a 52 mm filter size for the most part of the lens range. On a semi-tele like the 105, 52 mm is a bit overkill so you may have success and no vignetting with a 49 mm filter).
Hpe this helps.