That is a tough one as the case hardening is probably a metal stain of some sort which is a surface treatment. The real case hardening process requires all sorts of nasty chemicals, took time and expertise, and went out of favor decades ago. NEF probably went the stain route which is a surface treatment of the metal designed to look like case hardening but do not know for sure and probably no one in the current organization by that name could help you.
With the presumption that it is a stain rather than true case hardening, try the least invasive first as the procedure is the same regardless of the finish. Put a penetrating oil on the rust spots to loosen the rust and float it. Let it sit a time, then wipe it off with a rough paper towel (the ones you find in bathrooms). Kroil is probably the best for this but other penetrating oil will work. Then, try more complicated methods, I have use a soft rubber eraser on very minor rust left after the penetrating oil bath. I have also had luck polishing light rust out with Flitz metal polish after the oil bath.
If scrubbing the rust is necessary, use bronze wool (the right kind of chore boy pad) with plenty of light penetrating oil such as Kroil to float the rust (rust is abrasive and will scratch the finish--causing dull areas around where you scrubbed without a penetrating oil to float. Apply oil frequently, scrub, and mop up the contaminated oil on a regular basis--like a dental hygeniest. Blue Wonder rust remover also works to remove rust but not sure if the Blue Wonder would react with case hardening. If you decide to use it, try it on an area that is not seen.
Steel wool would be the last resort here and you would need to use the finest grade (0000 at the very least), carefully limit your scrubbing to the rust spots themselves, use penetrating oil procedure listed above, and hope that it did not remove nor dull the finish around those spots.
Might be a good idea to try out any techniques listed above first on an unseen area to practice before "going live".
