Yes, there are some Model 15s that have only U.S. rather than U.S.A.F. Some of those shipped to the Army, not the Air Force.
A bigger issue is the serial number on your gun, or, if I'm understanding you correctly, the lack of one. If it has been removed or obliterated you are holding what the ATF considers "illegal contraband." You may want to read this thread:
Aircrewman with obliterated number
Regards,
Kevin Williams