Notice and procedure when forgetting to include IP PIN on paper return, and why is the refund status checker using line 34 instead of 35a?

[big toes]

I forgot to include the IP PIN on the mailed paper return because I didn't receive it by post from the IRS.

After a month of mailing, I used the refund status checker, and it says a notice has been sent for verification, but I haven't received it yet.

What does the notice say and what's the procedure for verification?

The refund status checker instructs to enter the expected refund amount from line 35a but gave an error. I entered the amount from line 34 instead, which worked, but that's the refund amount before subtracting the portion to be applied to next year's tax payment, which I need to avoid quarter 1 estimated tax underpayment.

Does that mean they won't be applying the portion to next year's payment due to not having the IP PIN?