This particular key was made in 1926, and is known as the "original" model.  Production of keys of this type began in 1905, and are they still being made today.  Originally, they were used strictly by landline telegraphers, but once radio came into use, they became favorites of radio operators as well.  Today, their use is largely restricted to the amateur radio community.

With such a long production history, these keys are not especially rare, but this example is in exceptionally nice condition, and still has the original nickel plating on the operating parts, and the decorative pinstriping on the black finished cast iron base is also largely intact.

The secret of its operation is weighted rod (center left in the pictures) that is joined to the operating lever (with the black knob and paddle at the right) by a piece of spring steel.  As the lever is moved to one side, the rod vibrates and produces a series of dots using the contact just above the nameplate.  Dashes are produced by moving the lever to the opposite side, once per dash, using the contacts at the far right. 

Thus, to send the distress signal "SOS" (dot dot dot   dash dash dash   dot dot dot), a straight key would need nine operations, while a "bug" would need just five.