Ahoy Peter,
Stainless steel will corrode on the underside of a fitting. That is, the side away from the oxygen. I suggest you take the whole fitting out and inspect it. If it is OK to your satisfaction, then reinstall it. I did just that a few years ago just to ease my mind about it. Mine did have some crevis corrosion that appeared minor. A welder/friend concurred that it was OK. Had it been questionable, then I would have had a new one made. A relatively easy thing for a metal fabricator to do.
Good Luck, Dave