Contact: the Civilian Marksmanship Program, the US Army Ordnance Museum Curator, InterArms, Blue Sky Ordnance, Arlington Ordnance, Numrich Arms, (and any other known importer of surplus Garands), and Fullerton Arms, to determine if any have historical records of that SN passing throught their activity.
May want to have a law-enforcement officer run the SN against the national data-base for stolen firearms or those confiscated. Even a phone call and a well placed letter to the BATF may render leads.
If it came back into the country, there should be an audit trail somewhere. If it stayed overseas as part of a lend-lease or security assistance program, there may not be records except in some deep underground DoD archive.
The worst would be if it were demilled prior to salvage by the CMP or other surplus buyer, as detailed records may not have been maintained for individual weapons by the bulk demilitarization activities.