Sorry, but I'm stumped. I've owned Gradys from that era and they had the 1.5", threaded standpipe. It looks to me like someone replaced the factory fitting with this R.C. Marine 1/2" O.D. fitting. Doesn't seem like a 1/2" outside diameter standpipe could move much water.
I googled this brand, as I'm sure you have, and I couldn't find anything like it.
Thinking way outside the box, you might end up cutting some long notches in 1/2" PVC, maybe with a thin smear of silicone caulk for a snug fit?
Maybe someone did this, along with downsizing the livewell pump, to reduce flow so as to accommodate some really fragile baits?
Sorry, not the most helpful reply for a fellow Sailfish owner.