It's been quite a while since I've seen an OPM 1099 with UNKNOWN.  Is this a CSA 1099R or a CSF 1099R?  A = original annuity, F = survivor annuity.

If it's A they probably would have retired a long time ago and already recovered the basis so it's likely 100% taxable.  If it's F then the survivor steps into the shoes of the deceased and continues to recover the same fixed dollar amount of basis each year.

Box 5 though, has nothing to do with it.  All the box 5 numbers I see are Insurance Premiums (if you keep reading, the box is titled "Employee Contributions/Designated ROTH Contributions or Insurance Premiums).  This is where OPM puts insurance premiums.  Basis recovery is based on the 9b Total Employee Contributions which are recovered over the life expectancy of the annuitant based on the age at retirement.

For the insurance premiums, I always get the annual notice of adjustment which can also show things not included in box 5 such as LTC insurance premiums.  Even for clients who do not itemize, they can get a state tax break for these here in VA.

Rick

Edit: Another thought, is there a note at the bottom of the form about distributions to a former spouse?  That can happen sometimes too and leave you stuck with UNKNOWN.