Not quite right - as I remember it these four lines represent the first four notes of what we call the Dorian mode today.

Please understand, Lox, that I'm not point-scoring or playing Ancient Notation Top Trumps, I'm just trying to make sure that the information we circulate is accurate.

I've been back and checked and the illustrations in the PDF I quoted earlier, together with other sources such as Howard Goodall's book 'Big Bangs' based on / tied in with the TV series, and 'The Story Of Notation' by C. F. Abdy Williams, confirm my earlier statement of what the four lines represented.

Guido was quite an innovator, because he is also credited with inventing what became sol-fa (Ut - Re - Mi - Fa - So - La in his version, later changed to Do as in Ut, a deer, a female deer and the 7th Ti added), and also, as Foolestroupe has mentioned, the Guidonian Hand (whereby the fingers, spaces between the fingers, and finger joints become aide-memoires for notes and intervals. I'd never heard of him until I saw the Howard Goodall series, and I'm suprised he's not rather better known.