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