before Divino Afflatu, that is, before 1911, the Second, Third, and Fourth Sundays of Advent took precedence over all feasts except duplex feasts of title, patron, or dedication, and only in the churches so dedicated; other duplex feasts, including the Immaculate Conception
Aren't the 2nd, 3rd, and 4th Sundays of Advent semiduplex of the 2nd class, and isn't the Immaculate Conception a duplex of the 1st class?
It's a little complicated, but yes you are right according to
Divino Afflatu rubrics. However, according to the
Breviarium Romanum from 1763 that I'm consulting, the Immaculate Conception would indeed have been transferred to Monday because, as of 1763, the feast was only a duplex of the SECOND class, which yields to a Sunday of the second class in the Tridentine rubrics. In 1763, the only feast of the Virgin that was first class was the Assumption; the Purification, Annunciation, Nativity, and Conception were all duplex second class.
The only feasts ranked duplex first class in 1763 are: Christmas, Epiphany, Holy Thursday, Good Friday, Holy Saturday, Easter Sunday, Easter Monday, Easter Tuesday, Ascension, Pentecost, Corpus Christi, Nativity of St. John the Baptist, Ss. Peter and Paul, Assumption, All Saints, proper Dedication of a Church, and the Patron or Title of a Church. From this list, only the Dedication, Patron, or Title would occur with the Sundays of Advent, so they are the only feasts in 1763 that would have outranked the 2nd, 3rd, and 4th Sundays.
I'm not sure when the Immaculate Conception was elevated to duplex first class; it was a day of precept even as a duplex second class.