I have any number of older US OS, that have those NSS or printer stamps on the back, and none have bled thru (tho if heavier ink was used, i could see that showing thru on the front, even from back on the day it was first stamped).
Do you plan on backing these? As Steve mentioned, too, if so, then likely, the backer would either sand those off, or the stamp mark might wash off, or lighten in the bath.
Sharpie, tho, is a different critter. I know Charlie posted elsewhere, saying that he had had no success in removing any kind of sharpie writing or marks. prior to backing a poster.