Claudius. A.D. 41-54. Æ sestertius. 24.08 gm. 35 mm. Rome mint. Struck A.D. 41-42. Laureate head right; TI CLAVDIVS CAESAR AVG P M TR P IMP; c/m: ROB- in incuse rectangle / EX S C / OB / CIVES / SERVATOS in four lines within oak wreath. RIC I 96 (cf. footnote and p. 11 for discussion of countermark). For c/m: Pangerl 23, note 150. Very Fine; rich tan patina. Bold good style portrait; clear countermark, carefully placed to not affect portrait. Rare.

Ex Davissons Auction 28 (December 2009) lot 113.

Applied as a mark of validation, the ROB countermark here is a very rare variety of the common PROB(atum) countermark, perhaps treating the P and R as ligate. For the only other example the cataloger has found see CNG 70 (21 September 2005) lot 645 (offered again as CNG e427, 22 August 2018, lot 422).
(C. Kraay, "The Behaviour of Early Imperial Countermarks," Essays Mattingly, pp. 130-131)