There are not a lot of PTO experts on here.  Maybe George will pop by but he has too many usernames for me to remember to tag the right one. 🙂

That said, sounds like that's the box I would check.  Most of the tech MegaBackDoorRoths I see now have automatic conversion turned on.  So the Box 1 and Box 5 are the same, no tax, just reported on 1040 5a with nothing taxable.  Before that there would sometimes be timing differences between contribution and conversion resulting a tiny taxable income amount.