For whatever reason, the OPM insists on putting "UNKNOWN" in box 2a. Just put nothing in box 2a (in place of "UNKNOWN"), and you will achieve the same result, vis-à-vis the IRS.
The IRS instructions to the entity that creates the 1099-R say, "Generally, you must enter the taxable amount in box 2a. However, if you are unable to reasonably obtain the data needed to compute the taxable amount, leave this box blank." See Box 2a in the 1099-R instructions. So you should leave this box blank as you enter it into TurboTax.
@busquets
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