Historical Perspectives

In Baen Books' The Technic Civilization Saga Vols I-III by Poul Anderson:

the three introductions referring to Noah Arkwright cover the period of the Polesotechnic League;
the two introductions written in the much later period of the Galactic Archaeological Society cover the Time of Troubles and the early Terran Empire;
the twelve extracts from The Earth Book Of Stormgate cover the Grand Survey, the Polesotechnic League, the colonization of Avalon and the Terran Imperial War on Avalon.

Subsequent periods - later Empire, Long Night, Allied Planets and Commonalty - are covered in Vols IV-VII but lack comparable introductions.

Unlike Arkwright and Hloch of Stormgate Choth, the Archaeological Society looks back on its subject period from a much later date. The Society refers to the Terran Empire as the First Empire, implying that there have been at least two although there is no indication of a second in the Chronology of Technic Civilization that is known to us.

A Second Empire could have coexisted with the Commonalty, but in a different spiral arm. If the name of the Society implies a civilization operating on the scale of the entire galaxy, then it is considerably later even than the Commonalty.