Here is what IRS has to say https://www.irs.gov/individuals/understanding-your-letter-6470

Here is what Spidel Publications has to say https://www.caltax.com/news/flash-email/2021-52-irs-sending-incorrect-and-incomplete-recovery-rebate... Even though this is primarily a California Tax based business they do very helpful federal news. Sign up for their Flash E-Mail at the link in the box to the right in this link https://www.caltax.com/news/flash-email/

They just sent out a Client Letter for attendees of their seminars to send to clients about the 6470. 

If you have not yet seen a Letter 6470, here is what is says (The date and client name is different on each, but the text seems to be the same)