Manafort of internal polling data from Trump’s presidential campaign to his Russian associate.

The new details provide a rare hint at what Mueller is examining in the final stretch of his nearly 21-month-old investigation — and underscore his deep interest in the Grand Havana Room gathering, which ended with the three men leaving through separate doors, as Judge Amy Berman Jackson noted.

Weissmann said in the hearing that one of the special counsel’s main tasks is to examine contacts between Americans and Russia during the 2016 race and determine whether Trump associates conspired with the Russian-backed interference campaign.

“That meeting — and what happened at that meeting — is of significance to the special counsel,” he said pointedly.

The hearing was held in a closed courtroom, and only a partial transcript wassession a “private” visit, during which he and Manafort gossiped about “bills unpaid by our clients” and the political scene in Ukraine, where Manafort had worked as a political consultant for a decade before joining Trump’s campaign.

'An unusual time'

There have long been questions about why Manafort would break away from his duties running Trump’s campaign to meet with his Russian employee, an encounter The Post first reported in 2017.

A former senior U.S. intelligence official, who spoke on the condition of anonymity because of the ongoing investigation, called the details about what occurred at the Grand Havana Room gathering “the most interesting and potentially significant development we have seen in a long time.”

Prosecutors have alleged that among the false statements Manafort made to investigators during his interviews in recent