With two drilled holes in each overflow you have options for Herbie or even Beananimal approach. Meaning, you can have 2 full siphon, with the remaining two a trickle or complete dry stand-by pipe. This should help with any potential water noise too.
I was thinking the holes on the back side were likely use for the returns? I do notice that they are not centered on the tank, but that's not a requirement for return plumbing.