A section 1231 sale would be from the sale of real property as opposed to a normal capital gain that would be from the sale of investments typically. Since you see entries for both of those, you should look on your Schedule K lines 8 and 9 to make sure you didn't report any sales twice.
The amended corporate return will look like the original return, it will just have the box in Part H #4 on the Form 1120-S page 1 checked that says Amended return.
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