How to report a stock sale with no clear cost basis?

I had owned some stock about 15 years ago and sold it a few years later.

 

As part of a class action lawsuit after I sold the stock, part of that settlement was awarding shareholders more stock during that time period. This was about 5 years or so after I sold it.

 

So I ended up with another bucket of shares. I held those for a few years and sold it in 2024. I have no idea how to calculate cost basis. To make it even more challenging to trace back, the brokerages I had when the second batch came in have been sold to other brokerages, so the records are not great (nor are mine). 

 

I can determine the value of the stock when I acquired the batch from the settlement, can I use that as the cost-basis? Any other ideas? 

 

Thanks!