hyperdrive in Poul Anderson's Psychotechnic History, we are told that the principles of the drive involve multiple dimensions and discontinuous psi functions. Thus, this is not the multiple quantum jumps hyperdrive of the same author's later Technic History.
In a future history series, the author might develop a near future period and a further future period, leaving a chronological gap between them. In Robert Heinlein's Future History, "Logic of Empire" shows indentured servitude on Venus while a Prophet gains support in the US whereas the succeeding installment, "If This Goes On -," shows a US theocracy that has caused a hiatus in space travel. We have not seen the First Prophet come to power although we are about to see one of his successors overthrown. Perhaps seventy years - as well as three "stories-to-be-told" - occupy the gap between the two installments.
The gap in Poul Anderson's Psychotechnic History is much longer, 545 years according to the Chronology. In "Brake," human beings have spread through the Solar System but there is turmoil on Earth. In "Gypsy," human beings are spreading at faster then light speeds beyond the Solar System but the hyperspace ship, Traveler, is lost and its crew have settled on an Earth-like planet that they call "Harbor."
I am currently rereading "Gypsy" and have not yet found any internal evidence that it belongs in the same future history. In fact, when does the FTL period of the history make clear that it connects with the STL period?
The Chronology reads:
2300 The Second Dark Ages
2784 Hyperdrive invented
Nomad culture develops
So what happened in 2600 that we are not told about? It must be something to do with the restoration of civilization after the Second Dark Ages but it is not the founding of the Stellar Union because that comes later:
2875 "Star Ship"
2900 Stellar Union and Coordination Service founded
The Coordinators begin less than a century after the Nomads.