The only difference between 1st generation w126 brakes and w123 brakes are the vented rotors and the length of the piston in the caliper to handle the thickness of the vented rotor of the w126. Otherwise the caliper is exactly the same. I put the w126 brakes on my wagon so I could have the vented rotors. It was a simple bolt on upgrade for the w123.
The rear calipers and rotors are the same and even use the same part numbers for both.