Numerical evidence, from computing the cases $g=2,3,\dots$, eventually $g\le 15$. I recall Carel saying he made the conjecture when $g$ was still pretty low.

That's not so clear. But that's a very mysterious property. The search for the meaning is on.

Numerical evidence, from computing the cases $g=2,3,\dots$, eventually $g\le 15$, and seeing the symmetry in the numbers $\dim R_g^n$. I recall Carel saying he made the conjecture when $g$ was still pretty low, maybe 6. For any $g$, there is an algorithm computing $\dim R^n_g$ in finite time, that Faber came up with.

That's not so clear. But that's a very mysterious property. The search for the meaning is on.