My own review of the lengthy Dillinger file revealed documents such as the above relative to the knowledge of Agents, police, Laboratories and others of the existence of "secret serial numbers" on various weapons during the '30s. In this case the weapon is a Super .38 Colt Automatic and as you'll note, while the gangsters obliterated the serial number on the frame of the gun, they neglected (or didn't know) that the "secret" number could be retrieved, thus assisting in tracing the weapons. While I'm not an avid gun collector or enthusiast, others have mentioned that Colt began the placement of these secret numbers back in the 1800's.
You can take note that the gun and holster are being forwarded to "the Division" which was what FBI Headquarters was known as in the '30s. The "Division exhibit" mentioned is actually the Dillinger gang weapons the Bureau was placing in an exhibit for eventual viewing by the public. If you've ever taken the FBI tour in Washington, D. C. you very well may have observed the two items mentioned here.
You can find more on Lebman in the navigation area. Thomas "Tommy" Carroll was an associate of "Baby Face" Nelson and others and had been involved in the killing of a San Antonio police detective while there.