I recently scooped up a bunch of missalettes from the 1960s. Perhaps one day I shall subject them to greater scrutiny.
I doubt they contain anything that will shed light on this question, though:
I have yet to discover a satisfactory proposal as to why the old liturgy in the vernacular (essentially the Missal of 1965) was not a practical option.
One similar unexamined topic might be other temporary and transitional liturgical forms, such as this one, from 1964. It already goes way, way beyond 1965, it appears:
There is some discussion of this strange entity here. I know nothing, unfortunately. Nada.