In ,
Nancy Young typed:

After your previous note I thought how about calling the people
who put in the pool. Might be a start even though I don't like
dealing with them.

After you wrote that it was part of a pool drain system, I tried doing a
quick Google search to see if I could find anything about them.

I know nothing about them, but here are two links that I found:
http://www.aquamasters.com/deckdrains.htm

http://www.aquamasters.com/deck-o-drain.htm


The second one looks like what you have, but it does say that on that system
the top is not removable.