Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
The Original Constitution of Rome
Let us now sum up the results. Sovereignty, as conceived by the Romans, was inherent in the community of burgesses; but the burgess-body was never entitled to act alone, and was only entitled to co-operate in action, when there was to be a departure from existing rules. By its side stood the assembly of the elders of the community appointed for life, virtually a college of magistrates with regal power, called in the event of a vacancy in the royal office to administer it by means of their own members until it should be once more definitively filled, and entitled to overturn the illegal decrees of the community.
The royal power itself was, as Sallust says, at once absolute and limited by the laws (-imperium legitimum-); absolute, in so far as the king's command, whether righteous or not, must in the first instance be unconditionally obeyed; limited, in so far as a command contravening established usage and not sanctioned by the true sovereign--the people--carried no permanent legal consequences. The oldest constitution of Rome was thus in some measure constitutional monarchy inverted. In that form of government the king is regarded as the possessor and vehicle of the plenary power of the state, and accordingly acts of grace, for example, proceed solely from him, while the administration of the state belongs to the representatives of the people and to the executive responsible to them.
In the Roman constitution the community of the people exercised very much the same functions as belong to the king in England: the right of pardon, which in England is a prerogative of the crown, was in Rome a prerogative of the community; while all government was vested in the president of the state.
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Reference address : https://www.ellopos.net/elpenor/rome/1-05-original-constitution-rome.asp?pg=33