INDIANAPOLIS -- Colts owner Jim Irsay made a major late-night acquisition at a hefty price tag.

Irsay's latest move wasn't football related at all. Instead, it is the newest addition to his growing historic collection: Muhammad Ali's WBC championship belt won in the fighter's 1974 "Rumble in the Jungle" victory over George Foreman in Zaire.

Irsay acquired the item for $6 million at auction after a bidding process that lasted until nearly 4 a.m. Sunday morning. It is considered one of the most treasured examples of boxing memorabilia in existence. According to Heritage Auctions, the belt is one of just two known Ali WBC belts; the other is held in a private museum collection and unlikely to be sold, according to the company.

The belt is just the latest piece of Ali memorabilia to come into Irsay's possession. He previously purchased the shoes worn by Ali in the in 1975's "Thrilla' in Manila," Ali's famous victory over Joe Frazier. And earlier this year, Irsay purchased the walkout robe Ali wore in his first fight after changing his name from Cassius Clay -- a 1965 rematch with Sonny Liston.

Irsay has compiled a historic collection of hundreds of items. They range from rare rock and roll memorabilia like the famous Black Strat guitar of Pink Floyd's David Gilmour ($4 million) and Beatles drummer Ringo Starr's drum kit ($2.2 million), to documents written by America's founding fathers. But the Ali belt represents one of the most expensive items Irsay has ever purchased.

Ali defeated Foreman in the 1974 fight but was given the belt retroactively when belts began being awarded in 1976. Ali defended the WBC title nine times before yielding it in 1978.