49ers sign center Marcus Martin

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In the end, the 49ers completed contracts with all of their draft picks before the calendar turned to June.

The club announced Thursday it had signed rookie center Marcus Martin to a four-year deal. Martin, a third-round selection from USC, is the 12th and final Niners pick to sign in the last 16 days.

The 6-foot-3, 321-pound Martin was a three-season starter for Southern Cal, notching double-digit starts at center and guard. It’s possible Martin could push for time at center, where last season’s starter, Jonathan Goodwin, remains an unrestricted free agent.

“Right now, he’s going to be in the understudy role,” 49ers coach Jim Harbaugh said of Martin last week, according to the club. “That’s the expectation that we put on him, to learn and get up to speed as fast as possible, and, put no timetable on it. That’s his expectation right now. Learn the offense.”

Martin, who does not turn 21 until November, is the youngest player on the 49ers’ roster.