OPM is not following the IRS instructions for 1099-R box 2a. "UNKNOWN" is not a valid entry in that box. It should only be an amount or blank. The IRS instructions for box 2a for the organization that is issuing the 1099-R say "if you are unable to reasonably obtain the data needed to compute the taxable amount, leave this box blank."
When you enter the 1099-R from OPM in TurboTax, if box 2a says UNKNOWN leave it blank in TurboTax. Don't enter anything. Later on, when it asks you if the amount in box 2a was used as the taxable amount in past years, answer Yes, even though box 2a is blank. It will use the amount in box 1 as the taxable amount.
The checkboxes in box 2b are specified by the IRS. TurboTax cannot add a checkbox that is not part of the IRS form. Anyway, another checkbox is not needed. If OPM would fill out the form correctly there would be no confusion.
