This rig has an (unused) booster - it also has an open/shut ball valve on a 1" PVC line - that valve is for the booster.
Why someone thought they needed to both turn off the booster AND close the line, I don't even WANT to know, but there it is.
After the valve, it goes to a female threaded (1 1/2" NPT) fitting on the wall.
In this install, it is the only return with a threaded fitting.
If you have a small-diameter valve, check it when testing a the booster.