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Made on a budget of just over $1 million and shot in less than a month, Rocky is one of Hollywood's biggest success stories - becoming the highest-grossing film of 1976, spawning a series of sequels and spinoffs, and going on to win three Oscars including Best Picture.
Though Sylvester Stallone's masterpiece has been viewed for years, there's still something even the biggest Rocky fan hasn't seen.
On Friday, the 71-year-old actor shared a rare photo from the film from one of its deleted scenes.
"Rare deleted scene from Rocky," he wrote - explaining that the footage itself had been lost forever. "Unfortunately the studio burned all the outtakes by accident and only a few pictures remain. "This is the moment before Rcky and knocks out Dipper (played by the excellent Stan Shaw) who threatens him and took his locker!"
The action star added a single caption: "KEEP PUNCHING MY FRIENDS."
Stallone reprised his role as the Italian Stallion in the 2015 spinoff, Creed - earning himself an Oscar nomination for Best Supporting Actor.
Though he lost the best supporting actor Oscar to Mark Rylance, he still won praise from good friend Arnold Schwarzenegger - who posted a message telling him, "Sly, no matter what they say, remember, to me you are the best. You were the winner, I'm proud of you."
Sly's brother Frank was a little less gracious, venting his frustrations about the snub on Twitter.
"The Academy should be ashamed of themselves," he wrote. "It's as clear as the nose on your face that Sly won. Mark Rylance couldn't even comb his hair for Christ sake this is the 3rd time Sly's gotten ripped off Rocky, Copland and Creed. New Voters."
Gallery: TV and movie cast reunions we love (Provider: GES) TV and movie cast reunions we love