It seems that the use of the 1962 liturgy by the SSPX and Ecclesia Dei groups ultimately originated in an arbitrary decision made by Archbishop Lefebvre around 1983. (The Mass he was saying himself.) Which is certainly a curiosity in the history of the Church!
I wonder if anyone has any ideas about this question, however, which I've never heard raised or answered. (But please don't bring sedevacantism into it as it would be generally anachronistic for the date.)
A historical question - is there an answer?
Imagine a well-meaning, traditionally-minded priest in the summer of 1969, who has decided he really cannot say the New Mass which is starting in Advent. So far, although with increasing abhorrence, he has adopted the changes of 1962, 1965 and even 1967. Although he's already in trouble with his bishop, he doesn't want to go as far as the Abbe de Nantes and he hasn't yet heard of Archbishop Lefebvre and his little group in Switzerland, but he might be able to set up a chapel with the help of some sympathetic laymen . . .
But what none of them can decide is what Mass to say. If the priest is only going to reject the New Mass, he should stay with the (appalling) rubrics of 1967. Yet he would like to go back to the Judica Me and the Last Gospel. And perhaps the Second Confiteor and the Leonine Prayers (abolished in 1960). So would it be very disobedient to go back to 1962 or so? And of course there's nobody to ask. The laws won't exist very soon.
But if he can do that, he thinks, why not go right back, to before those depressing Holy Week changes. They all seem to have been influenced by the liturgical movement, which has turned out to be highly suspect and certainly in the hands of modernists. But again, on whose authority? So he grits his teeth, follows his instinct, and does a bit of historical research. Reads the Alta Vendita, disagrees with a book by Jungmann, and takes the plunge - back to the Mass of 1954. And what a relief to everyone.
And this was the position and the reasoning of priests I met in the early 1980's, when I first became a traditional Catholic. None of them, to a man, would have anything to do with 1962 or 1955, but as far as I know there were no particular arguments. Certainly nothing doctrinal. But now we are stuck in the world of 1962. Can we get out of it?