I realise this is getting horrendously close to hair splitting, but the link on Roman Factions is talking about working in twelfths, for example. To my mind, that is more of the mixed unit idea. There is a fixed big unit and a fixed small unit. To fully get into fractions, both the numerator and denominator need to be free: so I could talk about 13/19th if I chose. So while the fractional system that it talks about is on the way to a full fractional system, it is not quite there.