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I acquired my Pierre Allain Décrocheur from Speleoshoppe around 1980.
My Pierre Allain Décrocheur is 55 mm. long, 208 mm. wide, 22 mm. high, and weighs 154 g.
This décrocheur consists of a spring-loaded hook mounted between two side plates. The front and rear plates are made from 3 mm. aluminum alloy. Each side plate has one 15.2 mm. carabiner eye, four 15.2 mm. and six 7.2 mm. lightening holes. Five 3.9 mm. stainless steel pins, one 8.1 mm. aluminum pin, and one 7 mm. steel rivet hold the plates separated by 13.9 mm. The tops of the plates close together slightly at the eye. The inner mechanism consists of a hook made from 5.8 mm. aluminum alloy, a stainless steel spring, a stainless steel sear, and a sear/spring mounting lever.
The front plate is stamped with "P.A" inside an ellipse.
Despite rumors that it is a medieval torture device, its intended purpose is to allow one to rappel down a drop on a single rope, then recover the rope by pulling on a thin cord. The device is rigged by attaching the main line to a carabiner and clipping the carabiner into the top hole. The hook is then set over the anchor carabiner, and a long thin cord is run through the anchor carabiner as well. After the rappel, a good hard tug on the thin cord releases a sear, the spring retracts the hook, and assuming that the thin cord is twice the length of the drop, the rope can be lowered to the ground. If the cord is left in place, the rope can be returned to service by other techniques (which make this device superfluous, since they could have been used in the first place), and one can ascend.
A much bolder (and not recommended) approach is to have the last person down unlock the décrocheur before descending. As soon as they take their weight off the rope, the spring unhooks the décrocheur hook, and the rope and décrocheur fall and hit the last one down on the helmet. Hopefully, they are at the bottom and not just resting on a thin ledge partway down.
I remember seeing a picture of another version of the Pierre Allain Décrocheur that had a pulley mounted on the side. I am looking for one of these for my collection, but I can't even find the picture. I don't think that I imagined it! If it was real, I think that saw it around 1980, give or take a couple years.