My 80's Mercedes have a fixed flange transmission output like my 64 & 65 A's. However, they use rubber flex disks for axial motion and maybe anti-vibration (not terribly reliable).
Does your mercedes have an independent rear suspension? I have never seen the rubber flex coupler attached to a rear axle that moved with the suspension. I know that if you do a hard reverse launch those couplers go boom and come apart.