> I think it's plausible but I have seen "anoctothorpe" around more often;

Really? You've seen "anoctothorpe" more often than "an octothorpe"? That would be very surprising, given their respective Google hit ratios, as quoted by Faldage above. I had never seen "anoctothorpe" until your initial post, and automatically assumed it to be a simple case of a missing space.