I have twice seen a beautiful diffusion made in the 20s, 30s, or 40s. What I have seen are small round filters (around 55mm) meant to attach directly to a lens. The filter is reminiscent of a Fresnel lens, in that it has a number of concentric rings, about 3/8" apart.
I just saw an example at an Academy seminar. The filter was displayed by the ASC's archivist, and it was part of the collection of Joseph Walker, ASC (It Happened One Night, and many more).
This filter provides and extremely subtle and beautiful softening of faces without extremeflaring and without very much sense of defocusing.
I would like to get a filter like this, preferably in a modern format (preferably 4" X 5"). I have used a filter that Schneider (B+W) makes, and it's very nice. But it is less subtle than the old ones.
Does anyone know where this kind of filter can be obtained or where it can be made?