Detective Work Through Time

In 1955, Manse Everard finds, in a collection of Victorian and Edwardian stories, a reference to an 1894 tragedy at Addleton, England, and the mysterious contents of an ancient British barrow. Our timeline contains that collection but not the London Times reports which Everard reads in a New York public library the following day. (Later along his world-line, Everard would be able to read Times reports on his computer.) The reports recount a death caused by a deadly emanation from ingots in a chest found in the barrow.

In 1894, Charlie Whitcomb uses a radiocarbon counter to date the chest to 464 A.D.

The Time Patrol investigates:

"It turned out that even the Patrol knew little about the dark period when the Romans had left Britain, the Romano-British civilization was crumbling, and the English were moving in. It had never seemed an important one."
-Poul Anderson, "Time Patrol" IN Anderson, Time Patrol (Riverdale, NY, 2006), pp. 1-53 AT p. 29.

Observations
(i) The English were moving into England! - and giving it its name.
(ii) It had never seemed important? It marks the decline of the Roman Empire and the origin period of the King Arthur story. Such stories are perennially important.
(iii) Again, it had never seemed important? Does it seem important because of Everard's and Whitcomb's discoveries? Then it is known to be important by the Patrol office in London, 1894, and thus to any other Patrol agent with access to the records of that office which exists from 1890 until 1910. Words like "never" have to be used with extreme caution in time travel narratives. The same point was made about another investigation here.