I used those mirrors for about 4 years and they never left any kind of mark. That knob bears on another piece of plastic and not directly against the truck mirror surface. The piece of plastic is formed to fit the contour of the trucks mirror.
I was quite pleased with the way they worked and didn't vibrate or come off yet they were very easy and quick to put on and remove.
I think they are the best of "inexpensive" add-on towing mirrors.
Barney
2004 Sunnybrook Titan 30FKS TT
Hensley "Arrow" 1400# hitch (Sold)
Not towing now.
Former tow vehicles were 2016 Ram 2500 CTD, 2002 Ford F250, 7.3 PSD, 1997 Ram 2500 5.9 gas engine