Biro pen, have you heard of a "Bic" type pen? Bic being the make, that is what I'd call a biro writing pen Jeff. Not sure of any other way of explaining it.
I agree with you writing on the back of something would not normally bother me either, but, I wonder if over time it will gradually show through more? therefore one of my posters I'm a little wary of where the writing on the back is just in case it would eventually bleed through, if I was sure this would never happen I'd happily just forget about it.
Hey John, yeah, i know the term 'bic pen,' so biro may be a UK term/name for that same style. Its like blue ball point pen, then, right? This is just my thought, but I think bleed thru happens when the writing or stamp is first applied-- wet ink stamp, fresh pen ink, that can then migrate into the paper fibers and work its way to the front.
I would think that after many years or decades, when that ink has long dried, that there would be little chance of it continuing to "seep thru." I could be wrong, tho, and maybe Dan from Backing To the Future will chime in and give us more detailed infor about this.
I know I have posters from the 30s and 40s that have NSS ink stamps and writing (including black grease pencil, ball point pen ink and lead pencil) on the back, and nothing has appeared on the fronts of these posters in all this time.