I acquired my Rack Jack from Josh Babcock in 2007.
The Rack Jack is 152 mm. length of 4.5 mm. (3/16") steel rod with a 200 mm. length of 1.6 mm. (1/16") connected to a hole in one end. The other end of the cable has a 9 mm. loop. Overall, the rack jack is 152 mm. tall, 6 mm. wide, and 5 mm. thick. Mine weighs 23 g.
There are no markings on the Rack Jack.
Josh created the Rack Jack as a tool to give the rappel rack user a better grip on certain types of hollow brake bars. The Rack Jack works with both open- and closed-frame racks, but not solid, thick-walled, or U-shaped brake bars. Install the rack jack by threading the rack frame through the cable loop so that the loop sits below the fourth bar from the top (for open-frame racks, put it on the closed side of the rack). When ready to rappel, insert the rack jack through the lowest (or second lowest) bar in use. The Rack Jack provides a better grip on the selected bar. When not in use, remove the Jack from the Bar to make the rack easier to pack.
The Rack Jack is not made of stainless steel, so it will rust. Cable rusting is a particular problem.
Josh warns against poking your eye with the Rack Jack. I agree, don't do that.
Personally, I don't think that I need the Rack Jack, but I've
carefully adjusted my caving racks so that the bars slide smoothly.
One of my favorite racks, the Howell-N-Mann Mt. Sira, has a longer bar that
serves the same purpose, so I accept the premise behind the Rack
Jack (the Jack will not work with Howell-N-Mann brake bars)