S&W puts key locks on revolvers, and apart from a few J frames, has done so for years. A revolver is referred to as a "no-lock" or "pre-lock" to signify that it was either made before the feature was introduced, or that it was made after, but is one of the ones without the key lock. A revolver with the key lock is easy to spot. The key way is just above the cylinder release on the left side of the frame.