re: Two questions
The notches in the tracks are from a break in the normal tread in the wheels which is where they were bolted onto the lander deck during cruise, e, d, and l. they were pyro-fired and seperated during stand-up.
If you go to www.ksc.nasa.gov and look at the multimedia section and look at pictures from way back when MER was being put together - you can see this section on the wheel tread sometimes.
One notch = one revolution.
Doug