The OP was asking a question that undoubtedly concerned a "claim of rights" issue, but since he wasn't asking about HOW to report a claim of rights - only asking about attaching a letter of explanation to an income tax return - I limited my answer to that issue.
As to how you go about reporting a claim of rights and taking a deduction or credit in the current tax year, this answer is directed to that question:
<a rel="nofollow" target="_blank" href="
https://ttlc.intuit.com/questions/3661571-in-2016-i-repaid-a-salary-amount-of-3-332-reported-as-inco...>
Just add "1" to the tax years being discussed there.
