Ah, I see. I assumed he meant biro or pen marks that might bleed through.
He also clearly mentions, "a lot of theater info..and other stamps on the back, we have to get rid of...." that could be on the back of a poster...
Pen and other writing that could bleed thru to the front, of course, is a way to proceed. But the NSS number stamped on the back of US OS should be left alone, unless a client specifically asks that it be removed.
