The ones clients bring me have a Notice date up in the top right corner.

IPPINs issued IN 2021 can only be used on returns EFiled in 2021, regardless of the tax year youre EFiling.

Have they tried the steps on the IRS website to retrieve it? Just to double check that it matches the one in the letter.
https://www.irs.gov/identity-theft-fraud-scams/retrieve-your-ip-pin