My pool has something very similar to this. We think the pool was built in the late 60s or early 70s. We do have a main drain at the bottom of the pool that is plumbed separately<and leaking ;(>, but like you there is a fitting covering another pipe within the skimmer box. I don't know where it goes. We can't access our box from the deck though; we have to reach into the pool. I haven't been much of a help, but it is encouraging to find someone with a similar set up.