Info about the most recent El-Nikkors (recently discontinued) is available at http://www.europe-nikon.com/category...d=22&catId=152. At http://www.photo.net/bboard/q-and-a-...?msg_id=00FZ8J I explain what the "N" and "A" designations mean on El-Nikkors. The German manufacturers use different lens names for different lens designs, Nikon just calls all of their enlarging lenses El-Nikkor -- the 50 mm f4 and 75 mm f4 are the oddballs, consisting of 4 elements in 3 groups. The remainder are 6 elements in 4 groups.
Info about Schneider lenses is available at http://www.schneideroptics.com/photo...oto_enlarging/
Rodenstock info is at http://www.linos.de/en/prod/obj_vergroesserung.html
What format are you buying the lens for?