This model barometer was typically used at airports to determine the sea level pressure to be dialed in to the altimeters on aircraft.  The elevation in feet can be set on the small scale at the bottom of the 6 inch dial, and the sea level pressure is then displayed.  This instrument has a red line on the main dial which suggests it was used in an industrial application.  I found it fairly inexpensive for that reason.  It works well, however.

From 1954, an excerpt from a Kollsman catalog showing the altimeter setting barometer is available for download below (6 pages):