Pads on a 126 are the same between brands of calipers ie Bendix/ATE. They are different between front and rear.

On the 126, wear sensors are readily available for a couple of dollars each at the local parts store.

Did Mercedes do something different on the 123?

I suggest buying calipers locally unless you are buying new (which is expensive). Shipping on reman returns and the convenience of local exchanges easily offsets any price savings. I'm planning on driving my current cars long enough to need warranty replacement.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.