The drain pulls up and moves to the side to stay open to drain. We can tell from looking at the other tub that it is missing a little piece of metal that stops it from closing when you pull up and turn it to the right. However, we can't figure out how to take out the drain. There doesn't seem to be any type of screw or anything.

Our faucet in the bathtub is Delta Monitor so we guess the drain is from Delta too.

Does anyone have any idea how to fix this problem and what to ask for to find a replacement?