I contact OPM and get this repeated dribble.
If your annuity started after July 2, 1986, a portion of each annuity payment is taxable and a portion is a tax-free return of your contributions to the retirement fund. Your retirement contributions are shown on the 1099R form we send you each January for tax filing purposes.
To assist you in determining the tax-free portion of your annuity, the Internal Revenue Service (IRS) offers Publication 721, "Tax Guide to U.S. Civil Service Retirement Benefits". This publication is available at https://www.irs.gov/pub/irs-pdf/p721.pdf
This info is what I read before that sent me down this rabbit hole. This is not an adequate answer.
The search for an I.R.S. and O.P.M. knowledgeable person is now underfoot. If anybody has links I would appreciate.
