According to this document, a veteran of the Lucchese Crime Family returned to New York in January 1967 after a trip to Europe. The individual allegedly met with associates of Marcel Francisci to plan an individual's prison escape. After submitting the plan to Corallo, he approved it.
The guy who contacted me thinks that the individual whose escape is in question is most likely the famous heroin chemist Joseph Césari.
The page may be available in the FBI Files thread on Corallo, but just in case, here it is:

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Some thoughts/remarks:
-I have absolutely no evidence that Cesari is the individual in question, but considering his importance on the international organized crime map, his connections to the Francisci family and the fact that he was actually in prison at the time, show that it could be him.
-Césari was arrested in October 1964 with 105 kilos of heroin and 68 kilos of morphine base. Jean-François Francisci, Marcel's nephew, was there. Jean-François's father, Antoine, was having marital problems and had seen fit to dump his 7-year-old son at Césari's house.
-Although it could never be proven, Marcel Francisci was considered one of the main importers of morphine base, which, in addition to the previous point, links him to Jo Césari.
-The Lucchese Family was one of the main families in New York involved in drug trafficking (Ormento, Tramunti, DiPalermo, Corallo himself, etc.). Does this link Corallo directly to Francisci or Césari? Maybe not, but there's no smoke without fire.
-As discussed in another thread with PolackTony (viewtopic.php?p=257333&hilit=francisci#p257333), Corallo isn't the first American mafioso that can be linked to Francisci, as Francisci was active in the Lebanese gambling scene in the late 1950s, where Sam Giancana was also active. Plus, Giancana was also active in drug trafficking at that time.
-If Francisci was indeed active in the drug business (and despite the successful defamation lawsuits filed against the press, there's little doubt that he was), this likely connects him to two of the major bosses of the American mafia, which probably puts him in a position to call in a favor to help Césari escape.
-Césari was sentenced to seven years, but the escape plan was never carried out, and he was released in 1970.
For those who want to get an idea and watch the guy's video, it's here https://www.youtube.com/watch?v=R1Jwj2X ... azenBreeds.
He talks about the doc for the first 5 minutes, the rest is about Césari (it's more or less the same as the article I wrote for my blog last year https://unehistoiredecrimeorganise.blog ... i-les.html).
Whether or not Cesari is the subject of this escape, it's still a significant connection between two of the leading figures in organized crime at the time.
However, while I knew that Francisci had connections and friends in the United Kingdom, I didn't know that the Lucchese Family might have had connections there.