Sean "Puff Daddy" Combs is backing up Fat Joe's claim that Notorious B.I.G. banged Anthony Mason's GF ... saying the mystery Knick in "I Got A Story To Tell" is in fact Mason ... 100%.
Combs called into "The Breakfast Club" when Charlamagne asked if the bombshell revelation Fat Joe made on ESPN's "Highly Questionable" was factual ... or Hip Hop fable?
Fat Joe tapped Mason as the NY Knick in the classic rap song -- in which Biggie rhymes about banging a guy's GF ... then robbing him when the BF walked in on it (tough night).