A large mirror attached to the side of a computer monitor. In states other than Montana, the mirror is used to know when someone is entering your cubicle or office. In these cases it would just be known as a mirror.
In Montana, the mirror is used for anal intrusion detection, because when you are in Montana you are always watching your back.
"My Montana Mirror saved my ass again last night on 2nd shift. I looked up and saw the janitor whipping it out behind me just in time. "
"Damn man, get outa my Montana Mirror you fag! I'm not a sheep. Get that Montana Stick outa my cubicle! I'm calling security."