Hello, my pool was built in 1967 and I have a unidentified pipe in the deck. It's located about 1.5 feet from the skimmer and the pool edge. It is flexible, about 2" with another flexible 3/4" pipe inside. There was a grey fitting that was pipe clamped with a white removable cap flush with the deck. The cap was disintegrated so that's why I dug it up to replace. One thing to note is all of the original plumbing PVC on the pool is grey and electrical was run in steel conduit. Not sure if they made a grey plumbing variety in the late 60s or they used electrical conduit for the water lines.