Unless I missed it, I did not see any mention of una cuм masses in the Letter of the Nine (which makes sense honestly).
Yes, the thrust of that letter was about other serious issues, which shows that the roots of much of what has transpired within the Society in recent years was present at that earlier time under ABL.
Making it about sedevacantism simplified it for public consumption and deflected inquiry into the other more serious issues.
A repetition of this strategy is again in play in the SSPX/resistance.
The "split" of 1983 had little, if anything to do with sedevacantism. Archbishop Lefebvre was, unfortunately, under the influence of people in his organization who wanted a "deal" with Rome. And he was being constantly squeezed by "Latin Mass' types (Michael Davies, Hamish Fraser, etc.) who, although they surely liked the beauty and magnificence of the Tridentine Mass, they understood little, if any, of the causes of the debacle known as the Vatican Council. These persons, which included Father Schmidberger and his cohorts in Econe, wanted to bring all SSPX mass centers into uniformity around the 1962 missals.
However, the American seminarians who searched out the Archbishop for training and ordination were NOT using the 1962 missals in their home parishes in Long Island, New York City, Connecticut, Ohio and other places. Our U. S. traditionalists priests had never adopted the Paul VI liturgical books of 1962-- they were all using the missals of the early 1950's!
Already, by 1958, Bugnini had forced changes through, based on his left-leaning French-based Liturgical Commission. And, in France and Germany, in particular, the changes to the missals by 1958 were even more extensive. Unfortunately, the Society, in it's seminary in Econe, had also adopted many of the changes. (Of course, even with these changes, the mass appeared to be the old time traditional, because the masses said in the parishes in France were so much more modern).
In the Northeast district, then, the priests made sure that we CONTINUED the use of the thoroughly traditional liturgical books-- the same books Fathers Kelly, Sanborn, Dolan, etc., had been brought up on. And, the Archbishop promised us in America that "we are not here to change your customs-- we are here to provide for you certainly valid priests." This was a very important commitment the Society gave, to entice American Catholics to support the fledgling organization.
But, once Schmidberger, Aulinger and others realized that they really, really wanted to make a "deal" with Rome, and to have a deal they wanted to be able to say "everybody is on the 1962 books of Paul VI", trouble started brewing. The District head of the SSPX Northeast District (Fr. Clarence Kelly) remained steadfast in not modernizing the liturgical books; and he was supported by the Seminary Rector (Father Sanborn), and most of the other District priests.
If it wasn't for the other major issue, I believe that Father Buldoc, the District Superior of the Southwest District, would also have held fast on this issue (since, later on, he became quite convinced of the necessity to stay with the 1950 books). And this "other" issue was the acceptance into the U S Districts of "pres-by-ters" from India, to help with the parish missions. Father Kelly and Co. refused to allow these un-ordained pres-by-ters into SSPX chapels; Father Buldoc bought the line from Schmidberger that these pres-by-ters were okay, and allowed them into the Southwest district to assist him.
It's kind of sad, but funny, how these very two issues are still with us today! Because the SSPX management did not handle them correctly (in the early 1980's), and nip the errors in the bud immediately, these mistakes will continue to haunt. Now we have the SSPX accepting pres by ters, as has been done in the Asian district now for years. And still the issue of Catholics not understanding why the books of 1962 should not be used.