3520 questions on timing

Quick question, foreign person died October 2020 , leaving real estate (outside of USA) to 2 people. They obtain in year 2021, proof their are the heirs of the estate and respective share each person gets as a % (not actual $). This is follows by fair value appraisal  within the first 6 months of 2021 to determine f/v at time of death (oct 2020). Based on the % and fair value, now we know whom gets what.

 

at what time 3520 needs to be filed - for tax year 2020 (time of death) in 2021 or

 

for tax year 2021 (time of determined % and fair value amounts) in 2022?

 

Please let me know your thoughts on this matter. Thank you