CIMRM 809 - Inscription on broken altar. Isca (Carleon), Britain
According to this site, the inscription is RIB 322.
"An altar was found in one of the sets of baths just outside the legionary fortress at Caerleon dedicated by a soldier of the Second legion."1
The National Roman Legion Museum at Caerleon plaque states: "To the invicible Mithras, the well deserving, [...]s Justus, [...] of the Second Augustan Legion, set this up." The Mithraeum would have stood outside the fortress, but its site has yet to be located.
I.e.: "To the unconquered god Mithras ...s Justus, centurion of the II Legion Augusta, dedicates this freely as is right."
John Edward Lee, Isca silurum, or, An illustrated catalogue of the Museum of Antiquities at Caerleon, Longman, 1862, p.11-12:
(J.J. is John Jenkins).