While I was compiling booklets for the Liturgy of the Hours of the Fourth Sunday of Advent (see my progress so far here), it struck me that for this Sunday, the Ordo Cantus Officii (n. 8) gives antiphons for the Benedictus, for Year A, B and C.
However, for each day between December 17 and 24, there are proper antiphons for the Benedictus, similar to the O Antiphons for the Magnificat.
Liturgia Horarum adds: "Nisi dicenda sit ant. Nolíte timére (21 decembris) vel ant. Ecce compléta sunt (23 decembris)."

Is it to be understood that on the Fourth Sunday of Advent, at the Benedictus, the antiphon is sung according to Year A, B or C, except on December 21 or December 23?