Thanks for the reply.
I knew about the disconnector, but it never hurts to keep repeating it (no pun intended). Might just keep somebody from getting hurt.
My 520 definitely has a recoil operated slide lock. If you rack the slide and drop the hammer on an empty chamber (or snap cap) it won't unlock without using the manual unlock lever. But give the business end of the barrel a bump on the carpet and the action opens right up. I haven't tested this with live ammo because I'm reluctant to break something and the gun has a great deal of sentimental value to me.
I've attached some pics that show how I THINK it works. Plz forgive the quality.
Here's my theory:
When the slide is moved fully rearward it contacts the mystery part (for lack of a better term) pushing it back which then releases the slide lock and allows it to move up, thus locking the slide. During recoil the part moves forward and engages a pin on the slide lock moving it downward, disengaging the lock.
If anyone can correct my interpretation or add something I missed, it would be appreciated.