More learned specialists will doubtless opine shortly, in the meantime a few throughts of my own (since the Binchester blog doesn't appear to take comments).
The final two lines look to be something like
[name of unit] cui prAEST [name of commander] praef eQVITVM
ie ...which is commanded by [name] prefect of cavalry
cf RIB 897 from Papcastle
ala Aug Gordia(na)...cui praest Aemilius Crispinus pref eq
and other examples (RIB 817,1585,1688,1976, 2149).
The upper line would give us ...SACER. This does appear as a cognomen, especially in the celtic provinces, but it's not really in the right place (who's name would it be? the commander's name should come later. if not part of the name of the unit, it should form part of the subject of the dedication).
Much more interesting would be SACER(dos), or some such, which is found in the imperial titulature of Elagabalus.
cf RIB1465 - dedication to the emperor recording rebuilding at Chesters in AD221.
Except the emperor's name - in particular the sacerdos...dei invicti solis elegabali bit - has been erased there, so this is only inferred... and there doesn't seem enough space here to fit all of this, however abbreviated.
