Your rear flange plates for the brakes were discontinued in about 1998 and superseded with the plates where the shoe pins are inserted from the back. The official way to replace your type of plate is to replace it with the newer version and you will also need:
24428129 gasket for rear brake carrier and
24445691 seal for rear brake carrier.
The seal goes in the rectangular slot, I think. Can't remember about the gasket. You will also need a new style brake shoe fitting kit 93174289 which is easily available.
The trouble is to obtain new flange plates of any description. They have been unavailable anywhere for several years. Including Cadillac dealers in the USA and Canada. And I'm not sure that any 3rd party supplier ever made them, for an Omega B.
We managed to fix an Omega B with your plates by tack welding the domed washers, for the pins, from a new style fitting kit onto the plate to repair the damage. Then re-fitting the shoes with the old style cross-head pins inserted from the front. However that won't work if there is not enough sound metal left. Repair outlasted the rest of the car and never failed the MOT.
To fit new flange plates you will also have to separate the driveshaft and rear wheel bearing.
There's not enough info in the German ebay link to be certain that they are the correct flange plates. It could be that they're from something else, perhaps a Carlton or Calibra and are similar enough to be used. Expensive but on balance looks like it's worth a punt. Send the flange back if it's not usable.
Pay by Paypal and you might be able to get your return shipping costs refunded, you have to sign up for it with PP.