Is this the first year your mother received Civil Service Retirement? If not, her previous tax returns may have a worksheet showing the plan cost and amount previously recovered.
If you can't find any information you can choose to make the entire amount taxable and file an amended return if and when you find the plan cost. You have three years from the due date to file an amended return and receive a refund or within 2 years after the date you paid the tax, whichever is later.
Here is the contact webpage for OPM Retirement Services.
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