Tell us about yourself!
In the comments I gave a link to the meter display with a photo; the web store is called 'banggood.com'. I've ordered from them before, and have found similar parts on ebay and amazon. Good luck!
BTW, the unit listed only has a range of 2.5 to 30 volts so wouldn't work for your 120 volt AC application. Maybe the site has other products that would work?
See the website link I listed for specs; in the comments a user says:"Used to continuously measure multiple symmetric AC/DC variable power supplies. Precision is good. Adjustment is possible through a micro trimmer I recommend."
if you just want to measure a voltage, these are inexpensive: http://www.banggood.com/0_28-Inch-2_5V-30V-Mini-Digital-Voltmeter-p-974258.html
Bridging the outer and middle pins is done as a "failsafe"; see the section of this article:"When a potentiometer is used as a rheostat, the“unused” terminal is often connected to the wiper terminal"http://www.allaboutcircuits.com/textbook/experiments/chpt-3/potentiometer-rheostat/