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Any smudges on the focusing screen will NOT affect the quality of the image taken unless it is so bad that you cannot focus correctly.
What might be a problem is that if the small liece of foam has rotted, the focal length via mirror onto focusing screen will be slightly longer than the focal length to the film plane. Not much but if focus is really critical it might make a difference.
I have heard of ground glass plates that you can slot where the film usually goes to get perfect focus, with the back of the camera open, then you remove the plate and load your film and hope that the focus knob is locked nice and tight.
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