See my last post concerning the nuts!!
As to the washers, I am familiar with why they have been recommended. I was just throwing out the possibility of less contact with the frame being a factor in any of this!!
And you are one of the experts!! Don't sell yourself short!!
I haven't checked my frame thickness yet, will attempt to find out. My low mount coupler has the front boxed in pretty tightly. Will try to find a spot for a caliper or mic to fit.
As you know, my problem of dimpling of frame by the snap ups may well have been caused by the new, wider 1200# bars binding in the narrow cams of my early HP DC. So far no problems with the cam brackets to the frame, no bolt loosening since new. They fit very well up to the frame, not a square corner, and the tighter inner radius matches the frame well.