I have an older inground pool. The sand filter has no numbers or markings on it. It looks to be like a Pentair Tagelus by the pictures online. It has a plastic lid on it that has a oring in between the lid and the fiberglass tank. they list a plastic lid wrench to remove the lid. my question is there a nut on the bottom side or is the fiberglass thank threaded inside? thanks in advance.