I am coming back to the questions raised by the filter and the filter tread on the Schneider XL / 5.6.
I had to get myself the information from our technician at Alpa.
First of all, all necessary parts for mounting the filter on this lens are part of the lens kit.
There is first a thin ring which needs to be unscrewed from the lens: from there one has the possibility to mount and screw 2 different rings, with different diameters (95mm or 112mm).
The 112mm ring allows to shift without getting vignetting, but the choices of filters with this diameter is rather small.
The 95mm ring will start to show vignetting at about 80% of the IC. These are approximations, since the thickness of the used filter can be different from one to another
Here some images explaining the whole.
Thierry