It is a bit baffling how the mounting flange can take a knock while still mounted on the camera?
Anyway, if it is just your mounting flange buy a new one on Ebay. They are sold for people needing a coded mount on lenses that haven't got one, and it is just a simple unscrew the original and screw on the new type job. They cost fractions of what a techie or Leica would charge and are perfectly serviceable even though you'll hear the 'it can't be good it's made in China' whine at some point. So you need a 35mm mount with the correct hole spacing for your model Summicron, and a small screwdriver crosshead #00. Set down your lens on a flat surface and take off the original flange and place the new one in the same orientation, tighten the screws very slowly a bit at a time, opposite to opposite, this centres the flange and stops it causing lumpy notchy focusing. And that's it.
The alternative is to remove the flange and file any burr on the 'U' notch. So long as you don't pick the lens up or turn the focus ring with the flange removed it really is just a lift off and screw back on operation.
Steve