OT Coin Toss = 49'ers Demise?

GW93 said

If the OT clock ran down to zero while the Chiefs were still in their drive, with San Fran up 22-19, what would have happened? Would the game have ended with the 49ers as winners, or would it have gone into a "6th quarter?"

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A 6th quarter. My question is if the chiefs take the lead in the 6th quarter, does that mean the game is over or do they play out the quarter? Also, after both teams possess the ball does it become sudden death where next score wins? Doesn’t seem explained very well in the rules.