Not really applicable to the 139 but from the 159 onwards rubber covers were used on all the Contax SLRs. These often look past their best and, unlike the leatherette covers on earlier models, can't be easily replaced. I've experimented with methods of cleaning them and think I finally have a reasonable solution.
If the rubber is just showing signs of turning light grey or white from age, cleaning with a cloth and some isopropyl alcohol (IPA) will normally bring back the original colour. A toothbrush is useful to get into corners.
If cleaning with IPA doesn't work, or if the surface has become shiny from handling, then I've found scraping the surface is the answer. I use the back edge of a knife - the sort where you break off the blade as it becomes used. While scraping, the surface will become even more white, or maybe brown if there's a lot of dirt as well. This is then cleaned off with IPA. It might need scraping more than once to get the cover completely clean. I've found it's usually easier to remove the cover from the camera first. This allows the edges to be done without risk of scratching the camera.
Below is the rear of the same camera after being fully cleaned. The back previously had a lot of shiny areas from handling.
A caution: Some models had a different type of rubber covering. The only one I'm sure about is the rear door of the Aria. It has a very thin layer of rubber that is not removable and is often worn through so shiny areas are actually the plastic underneath. Don't try this technique on such covers.