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Jerry, Some revolvers, most notably those made by S&W and Taurus, have a key like an allen key which you can use to lock the action precluding the weapon from being fired. This S&W does not have that safety feature.

S&W M642 5RD 38SP +P 1.87 model #103810 does not have the built in internal locking system

It is essentially an internal manual lock that is manually actuated by a key. youtube.com/watch?v=hh5CKoWYtN8

To clarify this gun does not have the built in lock

Some S&Ws have a built-in lock where you can lock the action of the firearm here is a article on it luckygunner.com/lounge/smith-and-wesson-internal-lock/