Berg's adjuster still has to be welded in.
You cut a slot in the torsion tube at the proper angle to release the center puck, then weld a curved, toothed plate onto the outside of the torsion tube over the slot.
Once that's secure, you install a new (supplied) center puck stud (replacing the original which is discarded), a corresponding curved, toothed plate with a center hole, rather than a slot, and a couple of locking nuts.
Get your suspension more-or-less where you want it, tighten the nuts and see where it sets. Adjust (again and again) as necessary to get it to where you want it. More or less easy to adjust, although sometimes I've found it helpful to use the hex wrench into the stud as a lever to get it to move. The good news is once you get it set where you want it, you usually don't mess with it again. I would also recommend one on each torsion tube in the front, but that's just me. some people (and Vintage) only put one adjuster one one tube, but I'm not a fan of that.
A bit easier are the adjusters for dune buggies that allow you to adjust height simply by turning a bolt, but I've never used that version - maybe someone else has.