It looks like the top of the trap, where the old waste/overflow connected to it. You will need to remove the thin-walled brass tube and remove the adapter where it appears to screw into the trap inlet. Then you can use the proper adapter to hook in your new waste/overflow. Personally, I would cut the trap of completely with a grinder and replace the entire trap, since your new tub waste/overflow may not match up with where the trap is now. Make sense?