Not all servos have the same type or size of screw. Metal splined servos are usually a machine screw thread, plastic are sometimes self tappers. Different size servos get different size screws, different manufacturers have their own ideas. Putting the wrong type/size in either simply does not work (screw either won't go in or falls out) or does damage, hence the question.
No direct experience with a Seawind, but a friend had one with a stripped or otherwise damaged arm which he replaced at fairly low cost from a model shop. I don't know if it came with the appropriate screw, but knowing the make and model of the actual servo involved would help the search. Somewhere like servocity might have the information needed.