What kind of drain assembly am I dealing with here? The two pockets for the screws to hold the strainer in place are broken and I'm wondering if just that piece (and whatever seal might be underneath the flange) can be replaced without messing anything else up. If I can unthread that, what is actually securing the drain assembly to the shower floor? I've been unsuccessful trying to find shower drain components that look like this one so I'm at a loss regarding how this all goes together. Does a replacement part even exist?