Although the 'Mazac' tab. brake shoe problem is common, I cannot say that I have seen this before on an SG1 margin.  Interestingly, there is a similar 'exploding Mazac' problem on the ribbon reverse arm of the Olympia Model 8 post-war.  The factory probably had no idea at the time that this 'easy to die cast' metal would do this in years to come.

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.