House in So. California, built around 1965. Doing a master bath remodel. Did demo of the stall shower and found this in the plumbing wall. I'm sure this is vent pipes, but I don't know why the cleanout opening (if that's what it is called?) that was in the wall.

What is this for? I will be replacing the shower drain and trap, and if I were to replace this pictured area (not sure yet) with ABS what is best to do here?

NOTE: I definitely need to do something about this area because that cleanout piece sticks proud of the wall stud. It didn't bother the previous fiberglass shower but bothers me now with where I'm headed.

Many Thanks, Clark

(PS, sent here by the great folks at the John Bridge Forum)

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