detonation damaging cyl walls

[elevensecgn]

I've torn down a lot of engines before, but never seen this. A buddies
motor was apart about 1000 mi ago for the flat cam teardown. Thoroughly
cleaned everything, did brngs, crank, TC, OP and a touch up on the valves
along with a few
other prev maint items. Reassembled, car ran great. End of story----
Oh no,
he has since somehow detonated it to the point of #4 fire ring exit to the
outside, and lifted the heads and blew out the graphite and composition
behind the fire ring on 4 other cylinders into the valley and gave it a good
milkshake. We do a complete teardown, thouroughly inspect everything.
Believe it or not the bearings, cam, lifters, everything looks great.
(Thank goodness I plugged the oil filter bypass on the previous teardown)
the screen and oil filter caught all the crap. "yeaaa"

Now the tops of the bores in all cylinders have "ring marks" like a very
slight depression in the wall (all the way around)right where the rings
would be at or nearTDC. I'm guessing severe detonation caused the rings to
flutter
at the top of the bore and cause this, but haven't seen this before.

Unbelievably there is No detonation damage to the pistons or ring lands, and
the skirts look fine.
A thousand miles ago the bores looked perfect and still had cross hatch from
the factory. They still do except for the above mentioned ring depressions.

This is a new one on me, anyone else ever seen it, any thoughts on whether
detonation did in fact cause it.

Thanks,

Dwight Hayden