It seems that I need one...
Numerich is out of stock of these things.

Anywhere else to check? I don't think Jack First carries stuff this new, do they? I am going to call S&W tomorrow, but I doubt they'll have this since no more steel frame autos are being produced- but I don't know. First I've fooled around with these new fangled double action autos in the parts realm.

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Lost it all in a boat accident

Since S&W used the same parts on several models, you can use one from a similar model.

While they don't list the 645 specifically, Brownell's has a lot of S&W safety parts:

Schematics for Smith & Wesson® - World's Largest Supplier of Firearm Accessories, Gun Parts and Gunsmithing Tools

Jack First does carry newer model parts.

need safety for smith 645....