Welcome to the forum. The gun is likely a gun made before 1957 when S&W starting stamping them with model numbers. If you give us details of the gun, caliber, barrel length and type of sights we can help you. Even better a few good photos would solve the mystery quickly. I have a feeling it may be a 4" 38 Special which was a common duty gun, also called a Combat Masterpiece which became a Model 15 in 1957.

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John. SWCA #1586