This ala served in Upper Germany until the civil war in ad69, where they were employed by the emperor Vitellius in his futile bid for supremacy. After serving for a short time in Italy, they were shipped to Britain sometime in the seventies, possibly in the train of Petillius Cerialis as early as 71.
"... At Ateste¹ they heard that three cohorts of the Vitellian forces and the squadron of cavalry called Sebosian had occupied Forum Alieni² and built a bridge over the stream there. ..." (Tacitus Histories III.vi)
The first record of this regiment's service in Britain aside from diplomata is an Agricolan writing tablet from Carlisle. Attested on an inscription recording restoration work on the bathhouse and basilica at Lancaster, during the Gallic empire of Postumous. The early third century altar from Weardale in County Durham which records the hunting exploits of a commander of the unit, may have been placed when on secondment at the nearby Binchester fort, although the unit are known to have been active in the north during the third century, as shown by stamped tiles from Brough-under-Stainmoor.