The Discovery ticket has always been booked in M class. The fare basis is MxDIS2 . For a extra charge of about USD200 you can book it in H class HxDIS2

( x is the season L,J,K,H ) . My experience is that it can be slightly cheaper if booked directly with QF on their own Nett fare rather than the published edition. The ticket can be extended to 32,500 miles for an additional free of GBP270

Dave