This damage is typically seen on the front elements and front retaining rings from "oops" moments. When I saw the damage I immediately contacted the seller and he issued almost a full refund minus shipping costs. SO I ended up with this lens for nearly free and 5 hours of labor. I had 3 options to repair this one. 1: forget it and let it be since it was outside the image circle for 4x5 negatives. This lens was designed to cover a 7" image circle for 5x7 roll film. 2: Use a lens vise and work it out. 3: Use the old school method of a hardwood stick with the lens in a cradle. The cradle in this case happened to be a roll of "expired" nuclear red duct tape. You use a piece of hard wood (in my case a broken drum stick sanded down to a chisel point) and a mallet to gently tap out the dent. I chose not to use a lens vise because I was worried that it would cause the hairline fracture on the aluminum housing to break out more. The barrel would have no support to prevent over expansion and would deform more and the barrel would break. It took me about 20 minutes of carefully tapping away to straighten out the barrel and retaining ring. As you can see too, this retaining element is nearly identical to the front retaining element however this one does NOT have a hole drilled 180 degrees out from the set screw. I was able to use a silicon disc to remove the retaining ring after it was reformed. The threads were damaged making it pretty difficult to get out. When using the disc, avoid applying downward pressure since it will only add more friction between the threads over a larger surface area. Instead, find a disc that fits inside the ring and push outwards while unscrewing the ring. I eventually had to resort to using a scribe in the set screw hole to completely remove the ring.
Here you can see how much I was able to restore. The housing did partially open up from the hairline crack but further damage was prevented using the cradle method to reform. You can also see some minor fungus on the element.
