For the sedans, the rear brakes remained the same throughout the entire production run of the 126. The front brakes, however, were enlarged on the 2nd generation cars from 14" to 15" (278 to 300mm) rotors with correspondingly larger calipers and (inner) dust shields.

For the SEC the picture is a little more complicated. The constants here are that the rear rotors were always the same as on other 126s, and the later SECs have the same braking system, front and rear, as the later sedans. But there were several peculiarities to the early coupes. The most important of these is single-piston floating calipers on the front instead of the familiar dual-piston fixed calipers. The early SEC rotor was already slightly larger than that of the early sedan, at 285mm. And the rear calipers are also slightly different on the early SECs.

You appear to be a 123 owner. If you're contemplating a brake upgrade, I have read that early 126 brakes are a direct bolt-on to the 123. While I personally can't vouch for that, I suggest you stay away from early SEC equipment as potential transplants.