Early SD and some W123 cars had solid discs all around. If yours is one of these, it propably was swapped at one point, as the SEC should definitely have vented discs. Early cars have the floating caliper (coupe only), with smaller diameter, but larger swept area pads.
Later cars (not sure of the transition year, but it is not a gen1 to gen2 thing), have the two piston caliper setup. It is the same as the sedan's, and easily retrofitted to early cars. I did it on mine with great results. You'll need the rotor, dust shield and caliper.
Furthermore the pad price is around half of the early, larger pad's.
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