If you have a prior basis, put it on 2023 Form 8606 Line 2.
If IRS has a problem with that, they will contact you.
In future, keep it up to date.
your basis will reduce the tax impact of converting your Trad IRA to Roth IRA in amounts that makes sense to you.
Note:
Form 8606 can be mailed by itself only when you are otherwise not required to file a tax return.
The proper way to file missing Form 8606 is attached to Form 1040-X.
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