Leitz changed the cosmetics of this lens (DR Summicron) at some time during the production run. Most notably, the shape of the focusing ring, the measuring unit of the focus length, and the distance markings were changed. The older version has distances marked either in feet or in meters, while the newer version has both. Optically there should be no difference between these lenses, although it might be that the coatings were changed a bit during production of this lens, I am not sure about that. Whatever version you get, make sure the glass is scratch free and without haze inside. The glass and coatings of theses lenses is very prone to scratches and pitting. Internal haze will sometimes only show up when looking through the lens against a bright light source (flashlight). Haze will lower contrast and make the lens flare when shooting against the light. While internal haze can be cleaned sometimes, there is no guarantee a cleaning job will save the lens. If you luck out, the glasses are already permanently damaged and nothing can be done. So better to get a lens that is free of haze right from the start.
One more thing, Make sure you get the "goggles" together with your lens, they tend to be scarce and pricey as a separate item, and you run the risk of ending up with goggles that don't fit your lens, as there is a difference in the mount of the goggles between the older and the newer iteration of the DR Summicron.
Excellent information on this topic can be found here.