Proof of tax payment to IRS

Hello,
10 years ago I was self employed in another country, that country is asking me for proof that I made a self employment tax payment to the U.S.. Under the tax treaty self employed persons could ecide which system to contribute to.  I choose US. I have submitted a copy of my tax return and a copy of the cancelled check but this has not cleared up the issue.  I was thinking about getting a transcript but I see on the IRS website that there are 6 years of records and not 10 years.  I got a statement of earnings from social security but that shows earnings not contributions.  Do anyone how I can prove that I made a payment for self employed tax?

I believe that statement from Social Security shows earnings that were subject to payroll taxes.  e.g. I was a high earner so my statement does not show my earnings, only that portion which was subject to Social Security payments. (The SS tax stops after a certain limit).