I checked with S&W when mine rusted, and learned they do not make stainless screws for J's. I removed mine and cleaned it well with a Dremel brush. I replaced it and put a drop of melted candle wax into the screw channel. The wax seals the screw head from any moisture but is easily removed and replaced as needed.

I had the same problem with the grip pin on my S&W 457.

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Retired Police, Life NRA