The Centennial has a small pin stored under the grip provided by S&W to pin the safety into the off position.
The 340pd is just plan and simple a puta to shoot. I don't care who you are, there is no fun with this little 11 ounce .357 magnum (weight/energy ratio is the highest for a production handgun, or at least it was when it was introduced).
I've been carrying j frames for nearly 30 years, starting with a model 40, then 49, then my first 340pd for about the last 10 years, before acquiring this one.
The 340pd goes to the range once a year and gets 5 rounds fired. Regular practice is done with an all steel gun, either a model 40, 49 or occasionally my night stand gun, a 638, also equipped with CT grips.
These guns do what they were designed to do. If I ask more of them, I'm wasting my time and ammunition.