rjtjt,
I did a bit of googling.....
Is the 660 manufactured to a TSO? (145/146 for eg) I don't think it is, because the 660 is only meant for VFR navigation. Therefore, you may be receiving the signal.
On page 6 of your manual there is a blurb on satellite information. Next time you turn it on have a look at the satellite numbers it is using to determine position. If there is a satellite number outside of, or way off sequence than the usual numbers then that could be the Aus SBAS satellite and you system is at least detecting it and could be receiving/using a signal from it.
I don't know the specific PRN number allocated to our SBAS trial.