Earlier DR's have a single feet or meter focusing scale, and clip on eyes with the E. Leitz enlarger condenser logo, as shown on the top photo of this page, and focus to 19." Later DR's have a dual feet and meter focusing scale, with a simpler Leitz logo, and focus to 20." The optics in both versions are generally believed to be identical. There may well be other variations as well. As pointed out to me by Marvin Moss, the ball bearing mount on the earlier lenses is larger than on the later lenses. This results in the earlier close up attachment with the Leitz condenser logo fitting on later lenses, but not vice versa.