If you already made the payment in the meantime, the IRS knows that you paid. The tax liability and payment information is aggregated in your tax account. You don't need to show the additional payment on the Form 1040X. Use the explanation area in Form 1040X to describe the situation.
If you are looking for what the IRS has in its records, you can request a Tax Return Transcript from the IRS. See here for more information and instructions. You can sign up for an online account, or request a transcript to be received by mail.
There are several different types of transcripts. For detailed information including most line items from your original Form 1040-series tax return as filed, along with any forms and schedules, request a Tax Return Transcript.
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