Possibly -Mu Metal is usually a foil, say 0.01" thick or thereabouts. That may be lead for vibration damping. I can't think of another plausible reason for it to be applied like that.
Yep, wheel balancing weights.
mu metal has about the same look. But sheets are some times rather thick (maybe 1/32 to 1/16 inch). I don't think that a foil would be as effective.