Kuyavia was northern territory of the Przeworsk Culture, associated with the Lugian Federation. Northern borders of the Przeworsk Culture were along the Vistula and Noteć (Netze) rivers. Kuyavia was endangered by hostile Suevi to the west, and by hostile Goths to the north.
When Domitian sent 100 cavalry to support the Lugians, it would make sense to send them to Kuyavia.
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Archeologist dr Bartosz Kontny from Warsaw University said:
"Among many discovered metal items, there were also numerous hardware fittings made of copper alloys. It turns out, that these are unique embellishments which were part of Roman equestrian equipment and of uniforms of Roman legionaries.
Many of these findings are unique discoveries in this part of Europe [Barbaricum]."
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During Emperor Nero's reign, there was Roman expedition to the Baltic Sea for amber:
"There is still living a member of the equestrian order, who was sent thither by Julianus, the manager of the gladiatorial exhibitions for the Emperor Nero, to procure a supply of this article. Traversing the coasts of that country and visiting the various markets there, he brought back amber, in such vast quantities, as to admit of the nets, which are used for protecting the podium against the wild beasts, being studded with amber."
Source: Pliny XXXVII, 45. They crossed the area of modern Poland on their way there.
Amber was available mainly on the Baltic coasts of Eastern Pomerania and Prussia:
https://en.wikipedia.org/wiki/Amber_Road