The 3 no connects are for final calibration and test. The one screw inside ( the long ones) are removed, so that the current transformers are disconnected from the voltage sensing loop. Then the small terminals are used to apply the phase voltage and the meter power, while the high currents are supplied by a separate transformer that is only low voltage. That way you can test at the full 100A per phase, but do not need a 66kW load to do so, just a small transformer with a single turn secondary adjusted to give 100A through the shunt.
Dave was wondering as to the amount of input protection, but you have to consider that this meter is part of the distribution system, and has to survive lost phases, lost neutral and massive spikes along with massive long duration overvoltages. It has to work both during these and afterwards, and do so for at least 30 years without service. Thus the MOV's and the series resistors for them, along with the 3 phase bridge rectifier before a common mode LC filter feeding the power supply, designed to at least survive one mains capacitor going open circuit without degradation.
Separate power supply for the GSM module is to prevent people tampering by shorting the external power to the module thus turning the meter off. If that output is shorted the other rail will still provide power to the internals.
GPIO and relay outputs are protected as well, relay with a 275V MOV and the open collector optos with 60V MOV's.
On the current transformer side the compensation is done with a RC filter, not in the software. The RC also provides noise reduction. The ZD grey unit will be a small varistor to provide an AC clamp for overcurrent protection. The components there are exactly as seen in the datasheet with the added overvoltage device.