silvertip...revolvers with a factory nickel finish have an N (by itself, not inside anything) stamped on the lower left side of the grip frame. If the mark you are referring to is on the right side of the grip frame that is an inspector's/fitter's mark and does not indicate a factory nickel finish. S&W stamped these marks on the right side of the grip frame (I inside a rectangle, S inside a rectangle, N inside a rectangle, etc.). I believe each mark was specific to a certain inspector/fitter.

Many times, revolvers with a factory nickel finish will also have an N stamped somewhere on the cylinder (possibly under the extractor), but not always.

Bill