I have another Rollei 1.8 50mm in front of me. If I unscrew the whole front of the lens, it comes off and then 3 screws are visible, around the perimeter of the lens. Try unscrewing the whole front.
I had always thought the Contax and Rollei lenses just different in the mount, but perhaps the they are only the SAME in the optics and the style of the rubber rings. I saw a Contax 1.4 image on the net and sure enough, it does not have the screws on the focusing ring that are clearly visible on the Rollei version.