The Berwyn Group can research participant SSNs directly through the SSA or through commercial credit agencies even if you have limited data on file . The key data points that Berwyn would request would be Name, DOB, and Last Known Address. If the person you have no SSN for has been deceased for decades, they will not be active in commercial data sources BUT, Berwyn can likely correct the SSN be searching the SSA Death Master File (DMF).