I think that link pretty much answers it. Just simply looking at the scan from 'Le Triple Vocabulaire Infernal' shows that Page didn't make the symbol up himself, but that it was there in amongst the text way back in 1848. When you read through it it makes sense. The thing with symbolism is that you can take an existing symbol and add your own meaning to it. There seems to be sources, included in the link, that suspect the 'oso' part to mean 666. This is not necessarily true, or at least not necessarily the meaning that Jimmy has adopted. I always find that link to be pretty solid with the evidence that it gives. Obviously, Page didn't just make the symbol up...but it came from his interest in the occult etc.