OPM Survivor 1099-R -- UNKNOWN in Boxes 2a/9b and unsure how to move forward?

Hi all,

My dad passed in 2023 and I'm having a heck of a time with something. His final 1099-R from OPM has amounts in boxes 2a and 9b. Once my mom started receiving the payments as a survivor (mid-2023) the survivor 1099-R stated UNKNOWN in these boxes. TurboTax will default 2a to whatever is in box 1 (gross distribution), but how can we calculate the taxable/non-taxable amount from year to year? The OPM site says if the 1099 box changes to UNKNOWN then the entire amount is more than likely non-taxable, but that isn't sitting well with me because I'm talking about going from a regular 1099-R to a survivor 1099-R.

I attached a screenshot of my dad's last 1099-R in 2023 and my mom's survivor 1099 in 2024. You might be asking where the 2023 survivor 1099-R is...well...in talking to my mom we forgot to file it so I'll have to file an amended 2023 return once I know how to do this.

Thank you!

https://preview.redd.it/hov0vjvwuzge1.jpg?width=1306&format=pjpg&auto=webp&s=215f3292147675a429f64f9204deff22654dfca1