Could “Rocky Balboa” pack a surprise punch at the Christmas box office this weekend? Movie scored some knockout one-day box office numbers in its Wednesday debut for MGM, playing to $6.2 million. Written and directed by Sylvester Stallone, the newest installment in the franchise centers on the Italian Stallion’s return after a 16-year hiatus for yet another comeback. Warners also got an early jump on the holidays Wednesday, rolling out Clint Eastwood’s “Letters From Iwo Jima” in five locations for $20,522. Unlike its companion film, “Flags of Our Fathers,” which Paramount handled, “Letters” is platforming and will not expand more widely this weekend. “Rocky” expands from 2,752 playdates to 3,017 today, but the grizzled pugilist will face some stiff competish: Warner football pic “We Are Marshall” opens in more than 2,600, while Fox’s family-skewing “Night at the Museum” has booked 3,688. Opening in more than 2,200, over a crowded frame, will be Universal political pic “The Good Shepherd.”