|P P LEG XXII DEIOT PR LEG XX V V|
"Primipilus of the Twenty-second Legion Deiotariana, Prefect of the Twentieth Legion Valeria Victrix."
(RIB 573a; JRS lv (1965), p.221, no.5)
This would appear to be (part of?) the curriculum vitae of a commander of the Twentieth legion and if so does not prove that any part of Deiotarus' legion was stationed in Britain, merely that a former 'First Spear' of the legion was later posted to the Chester fortress as Prefect of the Twentieth.
Promotion from a primipilus to the rank of praefectus is, however, not normal. A centurion, even a 'Top Gun' primus pilus, would have come from among the lowest class of Roman citizens, the plebs, whereas a praefectus legionis would be recruited from among the upper echelons of Roman society and would traditionally have been a senator of praetorian status who had formerly served in the legions as a tribunus laticlavius.
The normal career path established during the Republic was:
Beyond the rank of cavalry prefect an exceptional man may have gone on to become a senator, but this goal was achieved by few. Given this information it would appear that the most obvious expansion and translation of RIB 573a is incorrect. However, the text can also be read:
'The Praepositus of the Twenty-second Legion Deiotariana [and] the Prefect of the Twentieth Legion Valeria Victrix [have made this].'
Although I do not know for certain, it is possible that this stone formed the base of an altar or statue to the goddess Concordia, a deity known to have been invoked on altars at Carlisle in Cumbria (vide RIB 964a; base; Britannia xx (1989), p.331, no.4) and at Corbridge in Northumberland (RIB 1125), both in places where detachments from two separate legions were housed together in the same fort. Evidence of earlier legionary 'punch-ups' perhaps?
These are my own thoughts on the subject, and, having never read the original article in the Journal of Roman Studies I would suggest that you - and I - try to obtain a copy before making any other assumptions.