Info about the most recent El-Nikkors (recently discontinued) is available at At 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

Rodenstock info is at

What format are you buying the lens for?