If you mean the curved line between the edge of the "recoil shield" and the end of the "bolt" then this is completely normal. It starts on the fitting bench when the fitter is "putting up" the revolver. "Putting up" is what S&W calls the process of initial assembly of their guns.

Why didn't you ask about this at the gun shop when you bought the gun?

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Gunsmithing since 1961