Here's your first research lead:

All this does is beg more questions.
"From Rome" - Who in Rome?
"To Secretary of State" - Which one?
"Vatican source expresses personal view next pope will be elected outside of conclave." - Really? Who on earth wants to research some unknown person's personal opinion?
"Speculating on Pius XII's successor, source said college may well chose an old cardinal..." - Do tell.
This is all there has ever been to researching this matter - a big nothing burger.
Not knowing he was perpetrating it, the pioneering faithful in the 60s sought to petition PPVI to fix the atrocities that were happening within the Church, but once they discovered that he was the perpetrator, there were all kinds of theories having to do with the real pope being taken prisoner and replaced.
What did that theory accomplish? Well it was another aid to the malevolent purposes of the enemy because what it actually accomplished was to add controversy, disagreements, factions, confusion, and disquietude into the chaos of the day among the already confused faithful. Seeing this is all it was doing, the theory died after only a few years - as should the Siri theory since, amazingly enough, it still only accomplishes the same thing.