It can be spelled either way, but the "modern" spelling is "el".
I am not certain when S&W started marking their nickel plated revolvers, perhaps someone with more knowledge can clarify, but S&W revolvers with model numbers and a good number of those that pre-date the model number system will have a "N" stamped on the grip frame of factory nickel plated revolvers. Those that were refinished by S&W will have "RN" stamped on the grip frame. If you are looking at a nickle plated S&W revolver, remove the stocks and look for either of these stampings.
If you see a S&W revolver that also has nickel plated trigger and/or nickle plated hammer, these are not factory plated. S&W did not plate hammers or triggers.
Also, S&W did not apply a copper plating to the base metal before applying the nickel. Some gun manufacturers and aftermarket finishes do have a layer of copper under the nickle, these are the nickel plated guns that are prone to having the finish peel if you use copper solvent gun cleaning solutions.