On January 25, 1966 the Consilium (the body charged to study the liturgy and suggest reforms, in the light of Sacrosanctum Concilium) of which Fr Bugnini was the secretary, issued the second of three letters it sent to all the presidents of episcopal conferences.
It covered a number of topics. The seventh was to do with 'the liturgical ministry of women at the altar.' As Bugnini summarises it: 'In some places such a ministry has been introduced, at times with a somewhat comical effect (female altar servers wearing surplices). The letter states that according to the legislation presently in force, such a ministry is not allowed.'
As so often with Bugnini, it is the footnote that is really revealing:
There was nonetheless to be an evolution. Requests came chiefly from communities of women and places where it was not easy to find a layman capable of proclaiming the readings in the vernacular, especially in mission countries. A statement regarding these cases was presented to the Holy Father on April 21, 1966. On June 6 the Secretariat of State replied: 'The Holy Father has deigned to decree that this favour may be granted in places where true necessity requires it only for the readings and the leading of the singing; it is to be done outside the sanctuary, by well-known women of adequate years and moral way of life, preferably religious women.' Individual concessions were granted in these precise terms. Then the requests became more widespread, and the GIRM made the concession universal but without changing the conditions: no laymen available; a suitable woman; office exercised outside the sanctuary (see GIRM 1970 no. 66).
The third instruction (Liturgicae instaurationes) of September 5, 1970, no. 5, introduced a further small extension when it said that women could proclaim the readings, the intentions for the prayer of the faithful, and the admonitions from a place where they could be easily heard with the aid of modern technological means (this was a roundabout way of saying they could go to the lectern). The instruction added that more detailed dispositions regarding place were to be given by the episcopal conferences. In the second edition of the Missal (1975) all the casuistic detail has disappeared, and it is said simply that the episcopal conferences can allow capable women to proclaim the readings and the intentions for the prayer of the faithful and can determine in greater detail the place from which this is to be done (no. 70). Thus there was a slow evolution that took in account the varying degrees of maturity and sensibility to be encountered in different places.
There is lots of interest here: I will pick up on just a few points.
The Holy Father (Paul VI) clearly thought that women should not normally be admitted to the sanctuary during the liturgy (the historic exception, of course, being the Nuptial Mass).
Bugnini and his like clearly thought that they should: this was progress, or 'evolution.'
Bugnini and his like won, via the typical 'hard case' to 'every day normal' slippery slope.
The issue was not addressed by proper process or scrutiny, but by gradualism.
Even at that time, there was no question of legitimising women or girls acting as altar servers. That, in fact, was done by the strange (but all-too-familiar) process of firstly acting in disobedience to the known liturgical law, and then raising a dubium, or question of doubt, about something where there was no doubt. The Vatican then gave the wrong answer (ie the answer that women were permitted to serve) when clearly from the Holy Father's statement quoted here, as well as from many other sources of text and tradition (dating back demonstrably at least to the late fourth century) that was not the case.
And I think it will be very hard to put the genii back in the bottle.
The FIUV position paper on male-only servers (in the EF) is well worth reading.