See response to other thread you posted
Being that you have purchased a firearms manufacture, you likely have a manufacturers FFL. I would suggest contacting your local ATF office and providing them the information on the rifle.
If the gun was obtained from another party by the former owner, they may have surrendered their A&D records to the ATF when they closed shop. There should be an acquisition record for that SN in the records they surrendered to the ATF.
I would start there for answers.....
Other reply deals with Lost and Stolen reports filed with the ATF and asking them to search for SN, model, caliber and maker to see if reported lost or stolen.