Just use the current date for Form 8606.
As for the address, IRS letters are generally very concise as to where to reply.
Don’t send it to Kansas City since, as you say, you’re not including a payment. If the letter isn’t specifically telling you to reply to the Atlanta address, but just telling you to file Form 1040-X if needed, then use the address in the Where To File section of the Instructions for Form 1040-X (Rev. September 2021).
Be sure to attach a copy of the letter you received, and you might want to attach a brief letter of explanation, just to tell what you’re doing and why. This way, even if your mail winds up in the wrong IRS office, it will be routed to the correct office.
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