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
While in jurisdiction they stood from the beginning on an equality
with the consuls and in the early stages of the conflicts between the
orders acquired like the consuls the right of initiating legislation,
they now received--we know not exactly when, but presumably at or soon
after the final equalization of the orders--a position of equality
with the consuls as confronting the practically governing authority,
the senate.
Hitherto they had been present at the proceedings of the
senate, sitting on a bench at the door; now they obtained, like the
other magistrates and by their side, a place in the senate itself and
the right to interpose their word in its discussions. If they were
precluded from the right of voting, this was simply an application of
the general principle of Roman state-law, that those only should give
counsel who were not called to act; in accordance with which the whole
of the acting magistrates possessed during their year of office only a
seat, not a vote, in the council of the state.(17)