40-70 year old ESOP stock cost basis that was gifted to grandchildren

My children were gifted stock from their grandmother that were purchased by my father in an ESOP between 1950 - 1983.  After my father died in 1994 my mother gifted them the stock.  They are thinking of selling the stock now, how do you come up with a cost basis for such a situation.  I called the employee investor relations that now handles the stock and they told me anything before 2011 is not on record.  All they can provide is the value of the stock on the day of the transfer in 1994. 

 

JIM