Question about Cost Basis not listed on a 1099B for a stock that was gifted

Found similar answer from 2020; however, want to make sure I'm reporting correctly.  In 2006, my mother gifted my son (age 3) her PSEG stock (my father was deceased) and I am still listed as the custodian (a total of 27.6 shares).  We've reinvested the dividends and in 2008 the stock split.  

 

In 2024, we sold 65/132 shares at 60.98 per share totaling $3755.58.  There is no cost basis listed on the 1099B.  The only paperwork I have (my mother is now deceased) is from 1985 and 1987 showing my parents had a total of 4 shares (no cost basis) to start.   I believe they reinvested dividends and probably bought more along the way.  

Do I put 0 in for the cost basis since I have no idea or does the IRS or???  

 

Thank you so much!