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I saw this modification at the PSC Pendleton County WV field house in the early 1980s, and took some photographs at that time. The photos didn't turn out. Around 1985 Bob Elron visited the Parks brothers and made this ascender under their direction.
This is a very nice modification of the Gibbs ascender. The basic addition is a stainless steel arm with a tubular steel pivot attached. The cam pivot hole was drilled out to accept the tube, and the side of the cam was milled to provide clearance for the arm (I'm told that the hard coated cam made the machining more difficult). The cam is mounted on the tube, and held in place with a stainless steel washer and external retaining ring. The other end of the arm was riveted to the shell. The result is an ascender that automatically aligns the cam hole with the shell hole, making rigging easier.
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Many of the early ropewalking systems used a shoulder-mounted Gibbs instead of a chest roller. This has the advantage of providing an automatic rest position - just sit down - although having the rope run slightly off-center challenges the emotional stability of some cavers. One disadvantage of this is that the rope drags on the cam shell, creating friction and soon wearing through the shell. Kyle Isenhart and Delbert Province (Nylon Highway #1) developed a simple solution: By making a "T-shaped cut in the shell and carefully straightening it, two "ears" are created that can be drilled to accept a bolt. A roller can be fashioned and mounted on the bolt.
My roller cam uses a 1/4-in stainless steel bolt and 5/16-in stainless steel tubing as the roller. I find that the roller usually doesn't turn, and recommend using a smaller bolt. On the other hand, there is no wear on my roller despite many miles of climbing.
The cam is modified in another way that is not visible in the photo. I took a large countersink bit and countersunk the pivot hole. This acts as a funnel to make it easier to insert the pin. It may void the manufacturer's liability (do so only at your own risk), but the ascender is easier to rig onto the rope.
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©2007, Gary D. Storrick