During the seventh BO7 campaign mission, Fracture, you'll be asked to hunt down a door code that 50/50 can't quite remember, but only remembers that it starts with 4-2. There's a short puzzle ...

While exploring The Base, you can find a locked door behind a vent that needs a door code to open. The Passcode digits are spread out across the Base in Datapoints that you can access as you progress ...