Check it with a magnet. 300 series SS is non magnetic. 300 series can look like it is rusting under a few conditions and will corrode through in salt water with a little stray current and in contact with something else that cuts off the oxygen that could get around it. Passivation could be done but it is not worth the trouble in this case (complete dis assembly), assuming that it is some sort of Stainless. Passivation removes the iron on the surface, works well on 15-5PH, 17-4PH and 18-8PH Knives and kitchen ware.

Make a new one, if you go to the trouble to take it apart go to the welding shop and get a piece of 1/8 316L welding rod and a 3mm die to put the threads on with (4-40 will also work)

Good luck getting it apart but I think that it can be done.

Later
Robert