Roman navy
Diplomas were issued during the Principate period (52-284 AD) to retiring veterans who had served in those corps of the Roman armed forces which enlisted "peregrini", that is, inhabitants of the Roman empire who were not Roman citizens (the vast majority of the empire's population in the 1st and 2nd centuries). Such corps were: the auxilia; Roman navy, the Praetorian Guard's cavalry ("equites singulares Augusti"); and the "cohortes urbanae" (the City of Rome's public-order battalions).