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
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
Manumission was unknown to the law of very early times. The owner might indeed refrain from exercising his proprietary rights; but this did not cancel the existing impossibility of master and slave coming under mutual obligations; still less did it enable the slave to acquire, in relation to the community, the rights of a guest or of a burgess. Accordingly manumission must have been at first simply -de facto-, not -de jure-; and the master cannot have been debarred from the possibility of again at pleasure treating the freedman as a slave. But there was a departure from this principle in cases where the master came under obligation not merely towards the slave, but towards the community, to leave him in possession of freedom.
There was no special legal form, however, for thus binding the master--the best proof that there was at first no such thing as a manumission,--but those methods were employed for this object which the law otherwise presented, testament, action, or census. If the master had either declared his slave free when executing his last will in the assembly of the people, or had allowed his slave to claim freedom in his own presence before a judge or to get his name inscribed in the valuation-roll, the freedman was regarded not indeed as a burgess, but as personally free in relation to his former master and his heirs, and was accordingly looked upon at first as a client, and in later times as a plebeian.(7)
7. Cf. I. VI. Dependents and Guests
The emancipation of a son encountered greater difficulties than that of a slave; for while the relation of master to slave was accidental and therefore capable of being dissolved at will, the father could never cease to be father. Accordingly in later times the son was obliged, in order to get free from the father, first to enter into slavery and then to be set free out of this latter state; but in the period now before us no emancipation of sons can have as yet existed.
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Reference address : https://www.ellopos.net/elpenor/rome/1-11-law-justice.asp?pg=16