I won this the other day. I already had a nice example of RIC 55 but found the officina interesting. It looks like a Gamma changed to a "B". The coin itself isn't too bad either, with a nice gold tone. Constantius II AE2. D N CONSTANTIVS P F AVG, laureate, rosette-diademed, draped & cuirassed bust left, holding globe / FEL TEMP REPARATIO, emperor bare headed & in military dress standing, holding labarum in his right hand, resting left hand on shield, two bound captives in Phrygian helmets kneeling, facing each other before him, ALEB in ex. Alexandria RIC VIII 55
I don't have enough ALEB coins to be of any help. The only one I have doesn't have a B that explains yours.
Would you please check the letter S on the obverse of the OP coin? It's in the upper right field, as part of the word TIUS. Engraver's design combined with worn condition, in my very humble opinion to conceive a clue.