I have never seen one of those where the full amount was not taxable.  If the client retired when the "3-Year Rule" was still in effect, that was so long ago the taxed contributions would have been recovered long ago.  I think this happened when someone retired on disability and the recovery period didn't start right away.  

You could check the 2023 tax return for that client to see how it was reported - the 1099-R was probably filled out the same way.

I would assume it's all taxable. Probably retired when the 3 year rule applied as Bob said.

All of my "unknown"s have died.  If you can get the retirement date (annuity start date) you can figure out what rules were in effect at the time.

I have also seen "unknown" in cases where there's an ex-spouse who has been assigned part of the benefits.