fyi; Dumoulin made a variety of guns but seems to be known more as a better quality maker than a junk peddler. You didn't give a desription or pictures of the rest of the gun but I would guess that if the peened hook is the only issue it is probably still a good buy.
Also try hanging the barrels by the hook and tapping them with a wooden dowel both up an down the length of each barrel to listen for a clear bell like ring. If a rib is loose it will usually be apparent with a buzz or just a "flat" tone at some point.
Sometimes the extractors/ejectors may vibrate and can falsely sound like a loose rib.
I've heard of hanging the barrels by a string wrapped around the ejectors as an alternate method that can reduce that issue.
I've usually been able to fiddle with their position (in/out/etc) to solve that or isolate it enough to tell what was happning.
The ring test is not absolute. I had a gun with nice Boehler barrels that rang like a church bell all the way down but had been rust blued and you could see the beginings of rib seperations on very close visual inspection.
BTW; Make sure that the barrels are held above a soft cushioned surface just in case they slip and fall.
