Sum it up in one sentence-Heller = primo.
In other words, there was no Primo boss in the original gang. This man is just a figment of Heller's imagination to carry out his plan to infiltrate gangs.
This can be seen from Heller's statement for the heads of the FBI. When Heller gave a report to the heads of the FBI, his intention was to show his deeper infiltration plan. To this end, he described many steps, including facial plastic surgery in stages, the physiological characteristics of slight limping, the change of native place, and the Italian accent. The most crucial sentence was that he said, "My name will become Primo Sparazza". So it can be seen from this sentence that this Primo is just an identity, which is Heller's identity when he was undercover in the gang. Of course, Heller had been active in the gang with this cover ID for a long time at that time, so it is estimated that this name is already known in the gang. Unfortunately, Heller's so-called deep penetration plan was regarded by the FBI leaders as a sign of his defection. At the end of the film, the deputy director said that "Heller can actually bring down the gang easily at this time", which can confirm this point. Think about it. If the leader thinks that Heller can destroy the gang, but he puts forward further undercover requirements, it is very likely to defect, so the FBI acted alone and killed Heller. And when he thought he was dead, he made up a story that he was killed by the gangster Primo to cover up Heller's real death. Of course, logically speaking, it is not difficult to guess that Heller=Primo is only known by the top officials of the FBI who attended the presentation, so it is not surprising that the outside world believes this statement.