I can’t remember what it’s purpose was, but it should have a mesh screen under the top level of the fitting. Some of the PE sets have this piece, but I didn’t think it was worth a $20 pe set for that one part.
The kit includes a circular cover that resembles a screen cover for that hole (not as good as PE but should be good enough to cover that hole). You may have missed it.
Anyway, I can see you are making good progress. Keep it up.