In today's EF Collect, Largire, the term propitius is very oddly placed. It can be forcibly translated as being in its usual function of an adjective in grammatical concord with the Deus or Dominus who is being addressed, and which thus invites an English adverbial rendering; "Mercifully" is the Cranmerian rendering in, ex. gr., the collects for Epiphany and Epiphany III in the Ordinariate Missal. (Interestingly, Cranmer omitted propitius in his rendering of today's collect, which in our rite is attached to the Ninth Sunday after Trinity for reasons which I explained on July 4). But its placing is just plain weird.
Sr Dr Haessly (sub Dominica) remarks that propitius is in an "unusual position", and wonders whether it should be taken with largire spiritum cogitandi or with largire spiritum agendi or as "a common element" with both. Sr Dr Ellebracht, whose promotor ad doctoratum was the well-nigh divine Christine Mohrmann (why do we hear so little from modern liturgists about these great women students of the Classical Roman Rite?) lists (sub voce) the word patterns within which propitius commonly occurs. "This adjective reveals an exceptionally strong tendency to be used in fixed expressions ... Thus we see how rigidly stylised the use of this adjective is. There is remarkably little variety in its employment." Today's Collect does not exhibit any of the four patterns she documents.
When one turns to the Verona Sacramentary (quondam "Leonine"), one finds that the text gives, not propitius, but promptius.
We instantly perceive the ease with which the m could be replaced with a tilde and thus become lost, leaving an incomprehensible proptius which would invite easy 'correction' to propitius.
The post-Conciliar revisers did not consider this elegant little collect good enough for Sunday use, but allowed it onto a feria.
And they gave the Verona text. In my view, rightly.