Looks like this may be it: http://www.brfares.com/#!rovers?nlc=I355
I would advise to look in the rovers & rangers menu under "EXPLORE WEST WALES".

You mention being 'very early' - note the time restriction: Not valid on trains timed to depart after 04:29 and before 08:45![]()
Be mindful that, if I remember correctly, ATW guards are no longer using Avantix, instead the new Fujitsu systems as discussed elsewhere.
It's not gone live over most of the network yet. As the official launch is somewhat delayed. Still Avantix in that part of Wales.
Guards down there have never had any trouble selling me that particular ticket.