Put it into a web search...there´s a couple of bits about it, mostly in French and virtually no pics.
This one is a big version..they appear to have been about twice the height of a man but I prefer big stone throwers. The range was apparently 180 meters with a 50kg shot and needed a crew of 8 to fire at a rate of 10 shots an hour.
Here´s a video of one firing. Thiers has a "running board" for the sling but some were just launched from the ground. The firing action looks pretty "violent", the counterweights wobbling all over the place.