Watch List notes:

The Roman Empire

Gaius, 37-41. Aureus 40, AV 7.83 g. C·CAESAR·AVG·PON·M·TR·POT·III·COS·III Laureate head of Gaius r. Rev. AGRIPPINA·MAT·C·CAES·AVG·GERM Draped bust of Agrippina r. C 5. BMC 22. RIC 21. CBN –. Calicó 327a (this coin).
Very rare. Several minor edge marks, possibly traces of mounting, and a graffito
below Agrippina's bust, otherwise extremely fine / about extremely fine

Ex Santamaria 14-15 October 1949, Magnaguti II, 507; Hess-Leu 23, 1961, E.S.R. 61; Lanz 94, 1999, 186 and Lanz 102, 2001, 473 sales.
When Caligula became emperor he did so as the lone-surviving male of the lines of Augustus and Germanicus; though his three sisters were still alive, he had lost both parents and both brothers during the reign of Tiberius. His father had died under mysterious circumstances and the rest fell during the family's contest for power against Tiberius and his prefect Sejanus.
Since Caligula's claim to power rested upon his membership in the Julio-Claudian family, early in his reign he tried to curry favour by taking a journey to the islands of Potnia and Pandateria to gather the ashes of his mother Agrippina and his eldest brother Nero Caesar (his brother Drusus Caesar died of starvation in prison, and none of his remains survived.) The seas were stormy, and the perilous nature of his journey only amplified the appearance of Caligula's devotion to the memory of his family.
As part of this programme of honouring his ill-fated relatives, Caligula issued this aureus with the portrait of his mother, who for the decade after Germanicus' death had been Tiberius's greatest critic, and his most persistent opponent. On this aureus her facial features are similar to those of Caligula – a feature that seems to combine family resemblance and a programmatic desire to show the kinship between the new emperor and his mother.