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I acquired my Ascender Bar from Gordon Hawkins in 2012.
The Ascender Bar is 790 mm. tall, 87 mm. wide, and 45 mm. thick, and weighs 1385 g. It consists of two ascenders mounted on a flattened tube. Each ascender shell is milled from an aluminum alloy extrusion and then gold anodized. The extrusion direction is oriented parallel to the vertical axis of the ascender, and contains two opposing channels. One channel is rounded and becomes the rope channel, the other is square and holds the cam. The rope channel is 15 mm. wide. A hole near the base of the rope channel allows mounting the ascender on the bar. There is a milled depression on the top of the frame near the rope channel, and a 2.6 mm. hole drilled through the depression.
The cams are plated skeletonized, reinforced stainless steel casting with a (4.3)^6(4) conical tooth count. The tooth axes are perpendicular to the cam face. The cam radius increases from 43 to 60 mm. over an angle of 43°, giving a 24° cam angle. The cam, cam spring, and a cam housing are mounted on a 5.8 mm. semi-tubular rivet. The cam housing is a piece of thin sheet metal bent to cover the top of the cam channel and serve as a spacer along the sides of the cam. The top of the housing is indented; this limits cam closing so that the teeth do not hit the inside of the rope channel. The cam safety is a piece of red anodized aluminum mounted on a 4 mm. stainless steel semi-tubular rivet through two lugs on the bottom of the cam. An anodized turned aluminum pin provides a thumb grip on the safety. The pin has a shallow U-groove for comfort. The normal action of the spring holds the safety against the cam. When the cam is opened, the shell interferes with the safety bar, thus preventing opening the cam. If the safety bar is moved away from the cam (opposing the spring), it will clear the shell and the cam will open. At full open the safety can be released and the spring will hold the safety against the back of the shell. This provides a means of locking the cam open.
The tube has a hollow oval cross section that is 35 mm. wide and 15.4 mm. thick. The wall thickness is 1.5 mm.The ascenders are bolted to the tube with 10 mm. custom socket hex bolts. There is a plastic washer between the ascenders and the tube, an the ascenders rotate freely. Two slots near the other end of the tube allow attachin4 a length of 25 mm. webbing.
The only markings are "06F" engraved dot matrix style on the rear of the end ascender.
What is this? Your guess is as good as mine, since I don't know for sure, and neither did the seller. In the listing he wrote the following:
ASCENDER DESCENDER BAR
Your guess is as good as mine on this item, I do not have clue what its used for!
There are 2 ascender type clamps on a metal bar with a webbing strap at other end. The devices are in full working order and I would recommend use with <12mm rope, they also fully rotate 360 degrees on the bar.
If anyone can tell me what this thing is, it would be a great help!
We had some good guesses but none of them really survived scrutiny. One of them was that it was part of an ascending system. Someone found a YouTube video that showed something similar. My reaction was, "The video is reminiscent of the Gossett System from the late 70s, without the box! I'm still not sure it explains the second ascender on the bar, though."
Gordon later wrote:
It might have been a window cleaning system, it turned up with a 3kgs stainless steel weight to tie on the rope.
My latest guess follows a different line. After thinking back to the old fence stretchers that I used back on the farm in my youth, I thought, could this be a device to tension tyroleans? Of course, after acquiring the CMI Rope Jack, whose purpose is known, it became clear that this might be an arborist tool intended for the same purpose.
Nick Hatch sent me some additional information:
I noticed that you have an interesting device described as an "Ascender Bar"  . I thought you might appreciate some more context, since the description is a bit hazy,
This appears to almost certainly be either an early prototype or homemade version of a rope tensioning tool, typically used for rigging purposes by arborists. You can see a video of one in action here: [link to a YouTube video].
The earliest mention I see of the device is in a forum in 2012, where Jack Holdway discusses development of the device . He worked with CMI to bring a commercial version of the device to market . I've included CMI's description of the tool from the manual  below.
There is a Gordon Hawkins in the UK who seems to be primarily interested in tree climbing & sales of tree climbing gear - if he's the one who donated the device, it's almost certainly for this purpose.
I did get my ascender bar from Gordon Hawkins in the UK, and Nick's information is undoubtedly correct.
One final comment: The cams and safeties on the ascender bar are undoubtedly made by ISC, as they match those on ISC ascenders. The extrusions appear to be ISC's as well.