taken from Wikipedia...
The Browning Double Automatic Shotgun is a short recoil operated  semi-automatic (Auto Loading) 12 gauge 2-3/4 chamber shotgun. This shotgun was offered in both aluminum and steel receiver configurations with the aluminum receivers featuring a variety of finish colors. The 3 versions offered include the Standard as the only steel receiver version, Twelvette, and Twentyweight which used the aluminium receiver. All Double Automatic's are 12 gauge. Various configurations were available for choke, barrel length, and barrel type. The firearm was produced between 1955 and 1971, with production volume of approximately 67,000.
This firearm features a unique loading port on the left side of the receiver. As the first round is loaded, the shell is lifted by a mechanism and chambered by the bolt; once the bolt is closed, the lifting mechanism returns to the resting position below the loading port, where the next round can then be loaded. Upon firing the chambered round, the shell in the loading port is automatically chambered. The total capacity is 2 with 1 chambered round and 1 in the loading port.
US Patent #2,586,509 issued to Val Browning Feb 19, 1952
Sometimes wrong, but never in doubt...