This is Mary Cromwell, born 1637, third daughter of Oliver Cromwell. The oddities of early Restauration England.
In 1657 Mary had married Thomas Belasyse, Viscount Fauconberg (later 1st Earl of Fauconberg). So here in 1663 she was Mary Fauconberg, or Viscountess Fauconberg. But Pepys refers to her by her father's name. Perhaps a subconscious sign of the overwhelming presence of the dead Oliver Cromwell in people's minds, even after his death? Cromwell had died only 5 years earlier, in 1658; Charles II had only taken power in 1660.
https://en.wikipedia.org/wiki/Mary_Cromwell,_Countess_Fauconberg
#Pepys #MaryCromwell #Fauconberg #OliverCromwell #Restauration