question for historians - germanicus and roman names

[caesar44]

hi all of you historians
here is a question with no answer in ancient books
as you know the roman name is divided - private name (publius) , clan name (cornelius) , family name (scipio) and sometimes nickname (africanus)
there were only several private names in rome - gaius , publius , marcus , decimus , manius , sextus , servius , mamercus , lucius , kaeso , geneus/gnaeius , titus , tiberius and maybe 1 or 2 more
in 42 bce there was a roman praetor called tiberius claudius nero , he had 2 sons - tiberius claudius nero (the future emperor) and ... anomaly .. nero claudius drusus where the hell his private name gone ???
drusus's son was the famous germanicus , he to did not have a private name
this is the first time in roman history
so , what was the private name of germanicus (not drusus , not claudius , not nero , not caesar and certainly not germanicus !!!)