Here is a large Claudius sestertius with a countermark from "the early years of Nero" [BMC p.xxxi] Very large at 36 mm and 24.44 grams with a strong and clear countermark. Reverse: OB CIVES SERVATOS in oak wreath Obverse: TI CLAVDIVS CAESAR AVG PM TRP IMP PP BMC 186. RIC 96, page 128. Countermark: NCAPR thought to mean "Nero Caesar Augustus Probavit". Of course, unworn coins did not need countermarks--they were still good and valued at the original value. However, some worn coins were revalidated with countermarks. If this one is judged by the beauty of its countermark, it is in high grade. Fortunately, most of the wear is on the reverse and the important portrait of Claudius is still good. Please compare to the $850 example on vcoins [$195 + 4 in the US] http://augustusmath.hypermart.net/coinlist.html
