The ones I've managed to find thus far:

F: "Happy Hour" Fare / Saver Fare (not offered via the GDS)
R, X, U, A, T, S: Saver Fare
M, B, N, Y: Flexi Fare
J: Premium Fare

There are probably others, but I haven't seen actual examples. Perhaps we can prevail on Virgin Australia to provide the full list?

If they do end up getting all the airlines under the one banner by the end of the year, I'd expect the above to change as well.