I'll say that clamp and washer were crushed by an overzealous mechanic. I've never seen a clamp stamped concave at the hole. If I did not feel like tightening that with a ratchet, I could make quick work with a 3/8" or 1/2" impact with an extension and easily crush the clamp and washer without stripping the pierce nut. Service mechanics grab air tools whenever they can, time is of the essence.