Please help identify this

Can someone please help me identify this object I found installed in a tub overflow hole. It is made of a plastic material, with a rubbery middle section that can be lifted up around the edge to make it look like an inverted umbrella when a strong wind catches it. The middle section is attached to the surrounding section by a small rubber post underneath. It appears to be some sort of 1-way valve… The whole thing is just sitting in the hole and not attached to the bolt holes inside. Judging from the dirt ring around the hole, it looks like the overflow plate was removed and this was installed. But what is it?

That’s a tub overflow cover.
I see them all the time around here.

It looks like a pregnancy prevention device from the sixties… that got stuck when the tub was overfilled…? Hope it hasn’t been there that long. If so, its time to replace it!

So, its purpose is to prevent an overflow or enable an overflow?

Might be this

Thank you, Perry! That’s exactly what it is. Somebody took this drain stopper and put it in the overflow hole.