I am currently using a 51.3 mm tamper which is a good fit in both the baskets. It was a 52 mm originally, and as such it was too big. I turned it down until it was a neat, but not tight fit, and that is what it measures now.
The crappy plastic tamper that came with my machine measures 50.4mm and it is a sloppy fit in theses baskets.
Perhaps there could be some variation in different lots of these baskets, and maybe Breville is playing it safe by recommending the 50mm size. Or maybe the person who answered your email just got it wrong - your guess is as good as mine on this.