If John Rao, John Vennari and the others would use Feeneyism ( no visible to us exceptions to the dogma on exclusive salvation in the Catholic Church) then the Letter of the Holy Office 1949 is a polite condemnation of Cardinal Richard Cushing, the Archbishop of Boston. Since being saved with implicit desire and in invincible ignorance (mentioned in the Letter) is not an exception to 'the dogma', 'the infallible teaching'( also mentioned in the Letter of the Holy Office 1949).
With Feeneyism ( no visible-dead) the Letter cites 'the dogma' whose text does not mention implicit desire etc. So the Letter of the Holy Office 1949 is in agreement with Fr.Leonard Feeney who was excommunicated for disobedience and was not asked by the Church to recant his traditional interpretation of the dogma on salvation.
John Rao, John Vennari and most of the SSPX members interpret the Letter of the Holy Office using Cushingism.
Also in the last paragraphs of the Letter of the Holy Office where Fr.Leonard Feeney and the St.Benedict Center are criticized , the SSPX assume it is for heresy and not disobedience. The text of the Letter mentions it was for disobedience.
So the participants at the 2013 Catholic Identity Conference would really being saying, through Cushingism, that the Letter has made an objective error in assuming that there are known exceptions to the literal interpretation of Fr.Leonard Feeney.Pope Pius XII was assuming we could see the dead-saved.This is the view of the SSPX and their traditionalist supporters.This is a factual mistake of Pope Pius XII according to them, since we canot see the dead on earth.In other words, they are saying that the magisterium has made a mistake.
With Feeneyism we see that the magisterium has not made a mistake. The error is there in the traditionalist's interpretation of the Council with an irrationality; being able to see the dead who are known exceptions to all needing to convert into the Catholic Church.
More party-line irrationality from Rao,Vennari and others: another politically correct conference