Underneath the sight glass threads, on the base. Not the ring gasket, and you can actually see the water line. I have a video, and will try to attach it. It is the actual base, where the water line is.
Tried attaching the 6 second video, but file is too large.
I wanted to update everyone on the multi-port. The new part was installed. Interestingly enough, I was able to peek inside the sand filter, (water up top), and really should have taken a picture. The water, even after one week on recirculate (solar cover on) was still crystal clear. What is more striking is how little debris there was on top the sand and inside.
I figured this could be shared because there are certain things I do through the course of the year that I feel may help, etc.
Now, there may be some channeling of the sand, but it is hard to tell.
For starters, I use pool socks in the skimmers, which trap a lot of debris through the skimmers, before it enters the plumbing, pump, and into the filter. I also use a DE alternative after backwashing immediately. Only backwash maybe 2X per year as pressure rises.
The last backwash (3X) is during closing in where I actually allow the backwash to run for like 5 minutes. Yes, this dumps a lot of water, but I usually bring it 1" below the skimmer. Start with the 2 skimmers only on the suction side, and and then close them off and work only from the main drain. After this, I place on rinse for about 1 minute to have the sand settle, and call it a day. Drain the water out and close the pool. There is no use of any "magic potions" or clarifiers, just the Fiber Clear (DE alternative) a few times a year. I followed advice from another member years ago, on what he does during the closing. I am 7 years into the pool, from 11-2013, full summer cycles.
The pool operates between 4-5 months before closing, and assuming if I lived in a State where the pump never shut down, things may be different. I feel that the 5 minute backwash yearly really makes a difference. Even the sand (after 6 years) looked like it was in good shape. Hope this helps.