If I may jump in here - as you probably know, the filters for the 37mm screw in at the rear of the lens (impossible to have flat filters in front of a fisheye lens). See attached lens diagram.
Originally Posted by Steve Smith
As far as I understand, the optical design of the lens was calculated to take into account the presence of a flat element at the rear (the filter), and if this is not present, it slightly changes the paths of the rays as they travel through the lens, causing them to converge at a slightly different point (in front of the film surface), hence the impossibility of achieving infinity focus. I do believe it's very slight, though (never bothered to waste a frame of 6x7cm film to see the effect) - almost looks in focus through the viewfinder.