Only 3 Zyxel models have it.
VMG8x24-B10A
VMG1312-B10A
VMG3926-B10A (never seen anyone with this device though).
Just an additional filter on the circuit board right behind the DSL port.
It gives that little bit extra stability, with what is my favourite chipset (BCM63168).
80 ES/hour should see you hold fastpath.