What is this thing on the slide?

I had been told this is where the "key" went for the Taurus safety system. But it doesn't really fit or do anything there. It *does* work on the hole on the back of the handle though. So what is this and what is its purpose? Looking at the parts list, it appears it may be an "extractor pin" down in that hole, but I don't know what that is or how it works or why there needs to be a hole with a groove leading to it.