You’re totally right. It’s a tough one. Aside from my cockup on using Biro/rollerball as one, I also should have stated that I PERSONALLY have a somewhat high degree of confidence. I guess I was just angry about the whole situation and rushed my post. Usually with pieces I have pulled out of buying where this issue has come up it has very obviously been rollerball (amongst many reasons, the ones you gave are usually key indicators).
However it was a gentlemen at the auction who knows this stuff far better than I do who confirmed to me the issue and to stay out of the bid (as I was not sure).
Typically with rollerball (aside from what you mentioned) it causes a very light bleed onto the back of the paper. I also know rollerball doesn’t fair as well with age etc
However just on the picture and now looking quite intensely at the differences with biro (previously rollerball issues I’ve looked at in auction have alll very characteristically been rollerball (I wasn’t actively assessing if a counterpart could have suited the writing)). But upon further research into biro, it’s not very clear which is which. In fact if I were betting on the picture alone I am inclined to say biro, which means the pen is period correct and nothing is wrong. However the good authority I have this information on, comes from a very astute watch specialist who could be wrong, but has not as of yet failed me.
I retract my earlier statement due to the way I delivered and phrased it. Poor form. hopefully this clears up my position. In hindsight I should have opened this as a discussion instead of a declaration.