Add this, from E.T. Salmon The Roman World: 30 BC to AD 138 (though he does not refer to it as a "Third Settlement"):
In the year 18 BC Augustus for the first time specifically named a successor by making Agrippa his partner in the Tribunician Power. The Imperium Proconsulare probably explains why Augustus waited until this year… never desirous of attracting attention to his possession of this supreme imperium, Augustus had never hitherto shared it with a colleague, for it would have been a constitutionally dubious procedure for him to delegate to an individual the sovereign power which was supposed to be conferred only by the People. Moreover if he nevertheless did so he would openly advertise that though he paraded the Tribunician Power, what he really valued was the Imperium Proconsulare. Yet his successor if he was called to office suddenly would urgently need the imperium: the Tribunician Power by itself without the backing of armed forces was not enough. Augustus therefore waited until his own tenure of the Imperium Proconsulare expired in 18, It was conferred afresh of course in the same year (as he knew it inevitably would be); but this time it was almost certainly conferred on Agrippa as well for a period of five years. This measure was put through quietly and inconspicuously, so inconspicuously indeed that the ancient sources do not explicitly mention it (footnote: Dio merely says that Augustus conferred upon Agrippa “other privileges almost equal to his own, including the Tribunician Power”. But in 17 BC Agrippa was exercising powers over both imperial and senatorial provinces: in other words he possessed imperium maius): the ostentation was reserved for Agrippa’s assumption of the Tribunician Power. Fundamentally, however, Agrippa’s Imperium Proconsulare was the more important, and it was to get him invested with it that Augustus had any constitutional arrangements at all made in the year 18….
Thus in 18 BC Agrippa became a co-regent with Augustus. If the latter were suddenly to die, Agrippa would automatically become Princeps. But the Principate did not now become a dual one with two equal rulers. Of the co-regents Augustus clearly enjoyed priority, for although Agrippa’s Tribunician Power, like Augustus’, was annually numbered, it had a five-year limit: Augustus’ was annual and perpetual.
Concurrently with these succession arrangements in 18 BC Augustus seized the occasion for a general reorganisation of the state. He conducted another revision of the Senate, with the intention of reducing its membership to three hundred… [but ended up naming] six hundred senators instead…
…[he] discharged other censorial functions in 18 BC. He refused to become a curator morum, but he did exercise a general supervision of public morals and conduct, and promulgated the Julian Laws. In effect these laws constituted a new code, for they made provision for criminal law and legal procedure as well as for moral reform…
The Emperor in fact became a High Court….
[Salmon goes on to describe the reform of the legal system and the introduction of the morality legislation that took place around 18 BC.]
Augustus could now claim that the wounds of the State, moral as well as political, had been healed. A new era of general prosperity and well-being could be anticipated. Therefore he had no hesitation in announcing that the Golden Age, which Virgil had so confidently predicted in 40 when the youthful Octavian was first emerging into power, had at last arrived. Nothing short of a magnificent celebration was worthy of the occasion, and such a celebration was duly solemnised in 17 BC at the Secular Games.
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