I assume doubles of the I and II class equals solemn feasts and double major and below (so, the ordinary double rank) equals double. The solemn feasts have full 1st and 2nd Vespers. They, in the 1911 reforms, always outrank Sundays in green and sometimes those of Easter and even Advent (Ss. Philip & James and the Immaculate Conception for example). Double feasts are either commemorated at first Vespers or might begin at the the chapter, from the appropriate common. They might have second Vespers insofar as there is no “V cap. seq.” (to use the St. Lawrence Press abbreviation) with commemoration of the preceding double feast; this happens when another feast follows that is also of double rank. In any case, doubles rarely have anything proper, and in the 1911 scheme only a ferial office. Doubles basically can’t outrank Sunday after 1911.

Think of it this way. Certain melodies are assigned to Marian feasts, some feasts and seasons have a unique melody and doxology used once (Epiphany octave), & some are for feasts of the Incarnation or other mystery and have a special doxology too. But what about things that aren’t proper or don’t fit those categories? Apostles & Evangelists would be a good example.

There’s no reason they can’t be, given that the Mass assignments are flexible, and liturgically speaking there’s no distinction, unlike in the 1969 liturgy where the second rank is still less full, so to speak. The distinction is mostly in what kind of feasts are at the highest rankings.

In fact, the Circumcision DNJC, the Holy Name DNJC, Candlemas (which is a Marian feast & one of Our Lord, just like the Annunciation), the Finding of the Holy Cross, the Transfiguration, St. Anne, St. Joachim, the dedication of the Archbasilica of the Holy Saviour (St. John Lateran) are all doubles of the second class, and they don’t fit categories like the feasts of the BVM (Masses IX & X), apostles (I had thought those were usually Mass IV), or martyrs (somewhere I picked up Mass V or VI was for them). That leaves Masses VII and VIII… perhaps those are suited for the historical commemorations like the Dedication of the Lateran Basilica or the Finding of the Holy Cross; okay, that still leaves the Dedication of St. Michael the Archangel as a D1C but…

I think the books would make clear on the day which melodies were to be used. It’s a bit of a mess in 1961, since so much was cut or changed in the Office.

Brief correction to the above: St. Anne only has a Creed if she is patron. St. Joachim had one due to the Assumption octave, so it isn’t entirely correct to say a D1C and D2C are indistinguishable. I’ve no idea why the feast has one in the 1954 Divino Afflatu rubrics on DivinumOfficium. So most D2C feasts have a Creed, but not all. I would say this is organic development and completely logical. They are, after all, part of Our Lord’s family.

Okay, another one:

In AR, there is a tone for the Benedicamus Domino, for use "In Festis Simplicibus".

The questions is: with the new 1960 rubrics, for what days / hours should this now be used, if any?

LU already indicates that the tone "In Festis Semiduplicibus" should be used for III class feasts, which is logical enough.

Thus, the thing I wonder about is whether there is any possibility that the tone "In Festis Simplicibus" ought nowadays to be used on days where there is only a commemoration (despite the fact that they remain IV class ferias), and / or also whether perhaps it ought to be used on the ferias of Christmas up to January 13th.

[This last because of the fact that Mass XV "In Festis Simplicibus" was relabled as "In commemorationibus et feriis temporis natalicii" - does this point to something, or is it a false clue?]

As far as I can tell, this phenomenon of having commemorations inscribed in the calendar merely as commemorations is a novelty, first occurring in 1955.

Up until then, you always commemorated a certain observance that happened to be losing out in precedence this year. Now, you have these floating, self-sufficient commemorations.

It is clear that in 1955 you would have treated these days in all ways as ferias, because real simple feasts still existed, and you would have used the tones "In Festis Simplicibus" for these.

Be that as it may, this situation did not last long: the simple feasts of 1955 became III class feasts in 1960, and in the process regained their tones formerly called "In Festis Semiduplicibus"; and this leaves the tones "In Festis Simplicibus" unclaimed, unless they can be given some new assignment.

Mass XV was reassigned, as we can see from the Missale Romanum.

The tone for the Benedicamus Domino, apparently not appearing in any books later than the Antiphonale 1949, is still anyone's guess.

[By the way, AR 1949 does have a section Mutationes in Antiphonali juxta novissimas rubricas that, although unlabeled, seems to be from 1955: it suppresses the rank of Semi-double.]

A small new bit of evidence I have just come upon:

In the online LU, p. lxxv-lxxvi, it is specified how to say the office on ferias occurring from January 2nd to 5th and also January 7th-12th, but as it doesn't say anything about the Benedicamus Domino.

[However, the other ferial vespers provided in the LU, p. 517, (Monday and Tuesday before Quinquagesima, apparently for Forty Hours) also do not mention the Benedicamus Domino. In this case, we know that the ferial tone ought to have been used, but to find it you would have to know to look at the foot of Mass XVIII in the Kyriale.]

So, the editors of LU circa 1960 were not much concerned about the tones for the Benedicamus Domino, such that they either overlooked this instance or thought that the user would need no guidance.

Either way, this helps give the impression that there would not have been any remarkable change in this matter.