Freedom of Information Act (FOIA) requests are great source of information on the past history of an NFA weapon. Other than tracing back the weapons history one past owner at a time, it is the only source of information. Some past owners of NFA weapons have preserved the original IRS/ATF registration Forms, but many have not. If the past history is unknown, filing a FOIA request with the ATF Disclosure Division is the first step in learning about your NFA weapon. I often encourage and assist owners of Thompson submachine guns to file FOIA requests. You don't know what you don't know. While the wait can sometimes be long, the cost is normally free. Yes, the past ownership information is redacted, but you can tell a lot of the past history from the dates, form types, titles and some of the government acceptance stamps. The earliest form is usually the most important. For example, the accepted definition of a Savage Commercial Thompson is a Savage Thompson that was sold to a law enforcement agency during World War II. If you find a Model of 1928 Savage Thompson without any military markings, no signs of military service such as inspector markings or arsenal/depot rebuild, no import markings, with the original dulite finish and matching receiver and frame, you may have a Savage Commercial Thompson. If there is no past history, how do you prove it is a Savage Commercial Thompson? A FOIA request may produce an IRS Form 1 showing the original registration was in the early 1940s. Many times, the words police or sheriff are unredacted. This is the perfect provenance of a commercial sale of a Model of 1928 Thompson submachine gun in the early 1940s to a non-military purchaser. Unfortunately, the IRS or ATF record keeping procedures were not the best and these early registration forms were lost (or at least not produced during a FOIA request). The earliest form may be a Form 5 from a government agency to a Class 3 dealer. This too is evidence of a commercial sale of the Thompson submachine gun in question. The forms usually associated with Thompson guns imported during the 1950s/60s are an IRS Form 2, normally from INTERARMS or similar company.
Again, you don't know what you don't know. Every FOIA packet is different. One tip: use the information directly from the current ATF form regarding manufacturer, model, and serial number when filing a FOIA request. It does not matter if you think some of the information is incorrect; what you submit must match what ATF has on file regarding the weapon.