Here is the BATF response to my lettter above.
Thank you for your recent inquiry to the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF). This is in response to your email, in which you asked about the possibility of ATF assigning a new serial number on a gun where the original serial number has been removed.
Under certain circumstances, an ATF-issued serial number can be assigned to a firearm when the original serial number has been removed. This process is facilitated through local ATF Field Offices. A listing of ATF office phone numbers can be found here.
Respectfully,
G. Kubert | Senior Industry Operations Investigator
U.S. Department of Justice | Bureau of Alcohol, Tobacco, Firearms and Explosives
Firearms Industry Programs Branch
99 New York Avenue NE, Mail Stop 6.N-512
Washington, DC 20226