It's to keep the pad from rattling, from what I understand.

The way the w126 cars have it setup, it's wedged in against the edge of the pad, and helps keep the pad forced against the caliper body. Again, if I understand it correctly, it should help keep the pad from jolting to the side when first applying the brakes, and may help to reduce brake squeal.

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...