See the discussion by K. Butcher in NC 2011, p. 75: ‘The type was first published in RPC I, when a single specimen was known (our specimen appears to be from different dies). There it was tentatively proposed that the letter B on the reverse was a date (‘year 2’), with reservations, because - unusually - there was no formula for ‘year’ preceding the number. A larger denomination with similar types, but with B at the end of the obverse legend, was published in the RPC Supplement I, S-3661A. The likelihood that B is indeed a date is reinforced by the new countermark on our specimen, which presumably refers to regnal year seven of Nero (ΝΕ[ΡWNOϹ] Ζ). If this interpretation is correct, the coinage with 'year 2' was produced in the first years of Nero's reign between AD 54-56 (Nero’s portrait is clearly a young one), and the countermark ‘year 7’ between AD 59-61.'