I managed to get hold of a first edition of O'Connell's Celebration of the Mass which he published in 1940. It is interesting for all sorts of rubrical reasons but I must say that I was caught by the following (O'Connell is discussing Custom):
"On the other hand it is very difficult to establish a real custom contrary to liturgical law (as found in the rubrics and in general decrees of the SRC) because of the resistance of the Holy See, owing to its desire for uniformity in matters liturgical. a) SRC in its decisions admits the force of custom only in minor matters and for particular cases (it seldom approves of a general usage contrary to the rubrics); b) each new typical edition of a liturgical book is prefaced by a decree approving its contents 'contrariis non obstantibus quibuscumque'; c) the volumes of the decrees of SRC are approved with a special decree containing the same clause; d) each new general, or equivalently general, decision of SRC has this clause also, and decrees of special moment add the words 'etiam speciali mentione dignis'.
Decisions of SRC which oppose existing usages at once abolish these - and this even if they are immemorial - for they prevent the consent of the legislator which alone can change a usage into a custom."
Now, there is a lot about the SRC not worrying too much about minor things: the use of a wooden stand instead of a cushion to support the Missal during Mass, for example; but we can establish from this that in 1940 the author of the manual which would become the standard for priests in at least England and Wales took as read that Rome wanted uniformity in matters liturgical and felt that it had the power to abolish anything contrary to any decision it took in this regard, no matter that the custom might predate Pius V.
This is not Bugnini's fault: at the time O'Connell was writing this Fr Bugnini was a curate only four years ordained and still not marked out for liturgical study.
This is yet another example of the fact that the worm had got into the apple before Pius XII became Pope. It is saying that the Pope can make any change he likes to the liturgical books simply because he is the supreme legislator, and that an appeal to custom cannot bind his hands.
These are deep waters.