Wondering if anyone happens to have an older Liber handy and can give a quick answer:

Around p. 124-127 or so, the Liber has tones for the Benedicamus Domino, and the LU 1962 shows the tone that is the same as the Kyrie from Mass IX under the title "IV. - In festis B. Mariae Virginis." This is all that is said about this tone.

On the other hand, as late as 1957, in Mass and Vespers we find this tone for feasts of the B.V.M. qualified with the note that "On Solemn feasts of the Blessed Virgin, the v. Benedicamus Domino is sung as on [Solemn feasts]." Which is what the Antiphonale specifies.

The question is, does LU ever omit this qualification in editions that pre-date the Mass and Vespers?

(If it does, then this would probably mean that the omission in the '62 LU was not meant to change the rubric found in the Antiphonale, since in Mass and Vespers we have this rubric still being observed.)