the disintegrating tab. brake shoes ! I have seen this on SM portables too - they have the same tab. brake. As a temporary measure, you could try filing the worst of the expansion off and re-fitting. You can also sometimes get away with having just three brake shoes instead of four.
via Tom Lucas (aka thetypewriterman), professional repair person at https://typewriter.boardhost.com/viewtopic.php?pid=32375#p32375
