John, I don't believe that Metrogons require a filter or piece of flat glass.
Of interest, perhaps a month ago I had the privilege, not joy, it was heavy, of holding a 180 Topogon in my lap. The outer-most element on each side was a piece of flat glass. Its then owner, who has friends at Zeiss, told me that according to them the flat glasses were part of the design and should not be removed.
Against that, I have a couple of Taylor Hobson lenses and an Elcan that were extracted from aerial cameras. All have pins for filter holders to attach to, and the two Taylor Hobsons arrived with yellow filters. All three shoot very well with no filter.
These lenses came in mounts that allow the lens to be collimated to its camera. Presumably any of them would have to be recollimated when setup changes, i.e., filter added or removed. And one of them shows signs of having been recollimated several times.