That's good to know about the pencil marks, I'd do that for light marks on the back, but all the ones I run across seem to be very heavy handed and on the fronts.
I have some stamp stuff that's been lightly marked by dealers or previous owners, and most of that erases just fine. Some of it I won't erase because it's too cool. Like a note on the back as to the date the original collector bought the item and which post office it came from. A couple of the ones I have are from the 1890's.
On the cleaning - Looks like the scratching is working fine. If color is coming off then you know that the ink on T201s may be water soluble and they can't be cleaned that way. Good info to file away for later. (Odd too, lithography relies on wet plates and oil based inks.)
Steve B