... and the fact that the Extraordinary Form is based on the 1962 Missal. (The Liturgy of St. Gregory predates '62 and hence the '62 Missal.)
It's my understanding that the AOC version of the Liturgy of St. Gregory does incorporate most of the reforms that were included in the 1962 Missal (in rubrics, reduction of octaves, calelndar changes) and that the Holy Week rites are those of 1955.
On the other side there are some Tridentine groups (The Society of St. Pius the V) who do use an older version of the Gregorian liturgy missal and even of the breviary (that of St. Pius X).
Am I right? Could you elaborate more on this?