Joe, I believe there are two options:
Last year when I contacted Schneider about the use of filters on this lens, I received this reply.
Schneider said that with the lens, you are supposed to get 2 rings, one has a outer diameter of 95mm and the other 112mm. You have to remove a ring on the front of the lens, which apparently covers the threads to mount these rings. When I asked about vignetting, I was told that @95mm you should be starting to see vignetting from a 5mm thick filter after about 80% of the image circle was used, with the 112mm ring they felt you should be able to shift all the way use 100% of the image circle, however they said this was dependent on the filter's thickness. Since I did not pursue the 28mm Schneider, I don't know if they include the rings with the lens or if they are an option at time of ordering.
So you can pick either 95mm or 112mm, 112 if you want to shift to the full diameter of the image circle without vignetting.
BTW, one of the best reviews of the 28's, Rodenstock and Schneider was worked up by Guy Mancuso over at getdpi.com a ton of feedback on both. Here is a link:http://www.getdpi.com/forum/lens-accessory-reviews/34991-rodenstock-28mm-hr-vs-schneider-super-digitar-28xl-2.html