That's called a semi-shoulder holster, worn crossdraw attached to a belt, the paddle raising the gun up under the inside of the arm. As you've found out, you can detach the paddle and wear the holster like a regular one, too. Clark/Bucheimer, Brauer, Jay-Pee and a few other older holster makers used to make them. That's the first S&W-marked one I've seen. Nice find. What is the design on the snap button?