Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
Among the sciences, that of jurisprudence acquired an invaluable basis
through the committing to writing of the laws of the city in the years
303, 304. This code, known under the name of the Twelve Tables, is
perhaps the oldest Roman document that deserves the name of a book.
The nucleus of the so-called -leges regiae- was probably not much more
recent. These were certain precepts chiefly of a ritual nature, which
rested upon traditional usage, and were probably promulgated to the
general public under the form of royal enactments by the college of
pontifices, which was entitled not to legislate but to point out the
law.
Moreover it may be presumed that from the commencement of this
period the more important decrees of the senate at any rate--if not
those of the people--were regularly recorded in writing; for already
in the earliest conflicts between the orders disputes took place as
to their preservation.(24)